I have a 2007 TrailBlazer LS AWD (4.2L L88 Engine w/ GU6 3.42 Front Differential) that developed a humming/groaning/growling noise in the front end. It sounded something like cardboard rubbing on tire tread or hitting fan blades. The noise was only audible above 25MPH, got louder with increased speed to 35MPH, then got quieter with increased speed above 35MPH (or maybe just didn't get louder, so it seemed relatively quieter as engine/wind/road noise got louder), and was only barely audible above 60MPH. The sound didn't seem to change noticeably when turning in either direction. There was no play or vibration in the steering wheel, but at 35MPH you could feel a vibration in the passenger floorboard and in the driver's pedals.
Apparently, this noise on a TrailBlazer is a typical symptom of a bad wheel bearing, and there are plenty of forum posts that mention how deceiving this noise can be. For example:
gmtnation.com
gmtnation.com
gmtnation.com
Unfortunately, I didn't check the forums before digging into it myself...
I put the TB up on jack stands, put it in 4WD, turned traction control off, and had someone "drive" it off the ground at 35MPH while I listened to various components with a mechanic's stethoscope to try to pinpoint the source of the noise. The sound was very distinctly coming from the front differential, and seemed to be loudest on the driver's side edge of the differential. With the TB turned off, I had someone spin the front driver's side wheel, and I could hear an occasional ticking noise in the driver's side of the differential using the stethoscope. There was no noise or play anywhere else, including in the axles/shafts and wheel bearings, and the sound was exactly the same with the TB in the air (with no load on the wheel bearings) as it was on the road. So I figured the problem must be a bad bearing in the differential, and I decided to rebuild it.
The differential is a pain to remove. The GM Service Manual tells you to remove the oil pan so you can drop the differential out the bottom. I instead removed the battery, radiator, alternator, etc, then unbolted the A/C compressor and moved it out of the way (without disconnecting the hoses), and lifted the differential out through the top, which was less work than removing the oil pan.
With the differential out of the TB, I couldn't hear any noise in the differential, even when spinning it fast with a drill. I suspected this meant the noise was coming from a shaft bearing (which doesn't spin without the shafts in place) rather than a pinion or carrier bearing (which does spin without the shafts in place). However, the differential spun very easily/freely with virtually no torque applied, when it should take at least 25inlb of torque to turn the differential due to the pinion/carrier bearing preloads, so the pinion/carrier bearings must also have been worn out, even though they weren't making noise yet themselves. After disassembling the differential, I didn't find any visibly damaged bearings. However, I figured I may as well just replace all of the bearings while I had it apart. I also disassembled the Intermediate Shaft Bearing (the unit attached to the passenger side of the oil pan) and found visibly worn bearings and gears inside that, so I decided to replace those parts as well. Unfortunately, I damaged the differential pinion while pressing the old inner pinion bearing off, so I ended up replacing the pinion and ring gear as well (the pinion and ring gear must be replaced as a pair).
I wanted to stick with OEM parts for everything (the differences in prices between OEM and aftermarket were not enough to warrant risking having to do the work again if the aftermarket parts didn't work well). However, the GM parts were ridiculously expensive. This differential was manufactured by AAM for GM. I found that searching for the AAM part numbers (which are listed on AAM's website) rather than the GM part numbers got me much cheaper prices for the exact same parts.
By far the biggest problem I ran into during the entire project was determining the appropriate size for the pinion shim in the differential. The shim size needs to be re-selected any time the pinion or inner pinion bearing is replaced. The GM Service Manual says to use a J-33838 tool to determine the correct shim size, so I purchased this tool. However, the tool was not very precise and gave readings that varied by as much as .006", when you ideally want a shim selected within +-.001". In addition, the median reading from the tool was .008" below the appropriate shim size as determined by trial and error with a gear contact pattern test (which is the definitive way to determine the appropriate shim size). I'm not sure whether I got a defective tool or the tool is simply not accurate in general, but either way the tool was useless and I ended up just selecting the shim by trial and error (which was a ton of work, as each trial requires fully disassembling and reassembling the differential, including pressing bearings on/off the pinion). For reference, https://www.cartechbooks.com/techtips/ring-and-pinion-gear-selection-for-optimal-performance/ has lots of great information on differential design/construction and gear contact pattern interpretation. When reading gear pattern charts, this particular differential uses "Face Hobbed" or "Two Cut" gears, not "Face Milled" or "Five Cut" gears. Note that the curvature of the teeth on this ring gear is backward from the curvature shown in most pictures/charts (drive side facing clockwise on this ring gear vs counterclockwise in most pictures/charts; perhaps this is typical of most front differentials, while most pictures/charts are of rear differentials?), so you need to carefully match up "Crown" or "Top" (peak of teeth), "Root" (bottom of valley between teeth), "Heel" (outer edge of ring), "Toe" (inner edge of ring), "Drive" (convex face of tooth), and "Coast" (concave face of tooth) between your ring gear and the picture/chart, and be aware that pictures/charts may appear mirrored relative to your ring gear due to the clockwise/counterclockwise orientation of the drive/coast faces of the teeth.
When I finally finished the rebuild and got the TB back together, the noise was still present, and still sounded like it was coming from the driver's side of the differential. By putting the TB up on jack stands with a block under one of the front wheels, I was able to confirm that the noise only occurred when the driver's side wheel was spinning. The driver's side half shaft had a bit of corrosion/pitting on it around where it sits in the differential bearing, so I figured either the pitting was causing the noise, or maybe something in the CV joint was causing the noise. So, I replaced the half shaft ... but the noise was still present, and still sounded like it was coming from the driver's side of the differential!
Since the wheel bearing was the only remaining rotating part on the driver's side that hadn't been replaced, I decided to replace the wheel bearing, even though there was no direct evidence that it might be the source of the noise. After that, the noise stopped! As best as I can tell, the sound/vibration was traveling through the half shaft and resonating in the differential, making it sound like the noise was coming from the differential even though it was actually originating in the wheel bearing.
As mentioned earlier, I did find evidence of wear in both the differential and intermediate shaft bearing, so I don't regret rebuilding them. However, it is rather aggravating to know that I could have fixed the noise in a tiny fraction of the time I actually spent on this had I replaced the wheel bearing at the beginning.
Bottom line: If you experience this noise, start by putting the TB up on jack stands, putting it in 4WD with traction control off, and blocking one front wheel then the other front wheel to determine which side of the front drivetrain is causing the noise. Then try replacing that wheel bearing, even if it doesn't sound like the wheel bearing is the problem. If that doesn't work, try replacing the half shaft. Rebuilding the differential is enough work/cost that it is not worth doing until you are absolutely sure that the wheel bearing and half shaft are not the problem.
Apparently, this noise on a TrailBlazer is a typical symptom of a bad wheel bearing, and there are plenty of forum posts that mention how deceiving this noise can be. For example:
Front End Growl
The problem is a growl from that sounds like the left front that starts at around 40mph and increases with some fine vibration in the steering wheel at 50mph. Turning slight left reduces and almost eliminates the noise/vibration. I just installed a new hub - no change at all. Differential is...
gmtnation.com
Noise on highway?
Hi everyone, I know noises are hard to diagnose or explain over the 'net, but I'll do my best: I've noticed a weird humming noise from about 35-60MPH (have not gone faster) coming from the drivers side front wheel area, as far as I can tell. It sounds very much like the noise you would get...
gmtnation.com
My TB suddenly sounds like it has mud tires
My 06 TB mostly gets driven on the weekends. Last weekend there was no issue, however yesterday we took it out and noticed a distinct howling, which sounds to me exactly like it has big mud tires on it. Seems to come from the front end. Sound starts as soon as we go over about 20 and gets...
gmtnation.com
Unfortunately, I didn't check the forums before digging into it myself...
I put the TB up on jack stands, put it in 4WD, turned traction control off, and had someone "drive" it off the ground at 35MPH while I listened to various components with a mechanic's stethoscope to try to pinpoint the source of the noise. The sound was very distinctly coming from the front differential, and seemed to be loudest on the driver's side edge of the differential. With the TB turned off, I had someone spin the front driver's side wheel, and I could hear an occasional ticking noise in the driver's side of the differential using the stethoscope. There was no noise or play anywhere else, including in the axles/shafts and wheel bearings, and the sound was exactly the same with the TB in the air (with no load on the wheel bearings) as it was on the road. So I figured the problem must be a bad bearing in the differential, and I decided to rebuild it.
The differential is a pain to remove. The GM Service Manual tells you to remove the oil pan so you can drop the differential out the bottom. I instead removed the battery, radiator, alternator, etc, then unbolted the A/C compressor and moved it out of the way (without disconnecting the hoses), and lifted the differential out through the top, which was less work than removing the oil pan.
With the differential out of the TB, I couldn't hear any noise in the differential, even when spinning it fast with a drill. I suspected this meant the noise was coming from a shaft bearing (which doesn't spin without the shafts in place) rather than a pinion or carrier bearing (which does spin without the shafts in place). However, the differential spun very easily/freely with virtually no torque applied, when it should take at least 25inlb of torque to turn the differential due to the pinion/carrier bearing preloads, so the pinion/carrier bearings must also have been worn out, even though they weren't making noise yet themselves. After disassembling the differential, I didn't find any visibly damaged bearings. However, I figured I may as well just replace all of the bearings while I had it apart. I also disassembled the Intermediate Shaft Bearing (the unit attached to the passenger side of the oil pan) and found visibly worn bearings and gears inside that, so I decided to replace those parts as well. Unfortunately, I damaged the differential pinion while pressing the old inner pinion bearing off, so I ended up replacing the pinion and ring gear as well (the pinion and ring gear must be replaced as a pair).
I wanted to stick with OEM parts for everything (the differences in prices between OEM and aftermarket were not enough to warrant risking having to do the work again if the aftermarket parts didn't work well). However, the GM parts were ridiculously expensive. This differential was manufactured by AAM for GM. I found that searching for the AAM part numbers (which are listed on AAM's website) rather than the GM part numbers got me much cheaper prices for the exact same parts.
By far the biggest problem I ran into during the entire project was determining the appropriate size for the pinion shim in the differential. The shim size needs to be re-selected any time the pinion or inner pinion bearing is replaced. The GM Service Manual says to use a J-33838 tool to determine the correct shim size, so I purchased this tool. However, the tool was not very precise and gave readings that varied by as much as .006", when you ideally want a shim selected within +-.001". In addition, the median reading from the tool was .008" below the appropriate shim size as determined by trial and error with a gear contact pattern test (which is the definitive way to determine the appropriate shim size). I'm not sure whether I got a defective tool or the tool is simply not accurate in general, but either way the tool was useless and I ended up just selecting the shim by trial and error (which was a ton of work, as each trial requires fully disassembling and reassembling the differential, including pressing bearings on/off the pinion). For reference, https://www.cartechbooks.com/techtips/ring-and-pinion-gear-selection-for-optimal-performance/ has lots of great information on differential design/construction and gear contact pattern interpretation. When reading gear pattern charts, this particular differential uses "Face Hobbed" or "Two Cut" gears, not "Face Milled" or "Five Cut" gears. Note that the curvature of the teeth on this ring gear is backward from the curvature shown in most pictures/charts (drive side facing clockwise on this ring gear vs counterclockwise in most pictures/charts; perhaps this is typical of most front differentials, while most pictures/charts are of rear differentials?), so you need to carefully match up "Crown" or "Top" (peak of teeth), "Root" (bottom of valley between teeth), "Heel" (outer edge of ring), "Toe" (inner edge of ring), "Drive" (convex face of tooth), and "Coast" (concave face of tooth) between your ring gear and the picture/chart, and be aware that pictures/charts may appear mirrored relative to your ring gear due to the clockwise/counterclockwise orientation of the drive/coast faces of the teeth.
When I finally finished the rebuild and got the TB back together, the noise was still present, and still sounded like it was coming from the driver's side of the differential. By putting the TB up on jack stands with a block under one of the front wheels, I was able to confirm that the noise only occurred when the driver's side wheel was spinning. The driver's side half shaft had a bit of corrosion/pitting on it around where it sits in the differential bearing, so I figured either the pitting was causing the noise, or maybe something in the CV joint was causing the noise. So, I replaced the half shaft ... but the noise was still present, and still sounded like it was coming from the driver's side of the differential!
Since the wheel bearing was the only remaining rotating part on the driver's side that hadn't been replaced, I decided to replace the wheel bearing, even though there was no direct evidence that it might be the source of the noise. After that, the noise stopped! As best as I can tell, the sound/vibration was traveling through the half shaft and resonating in the differential, making it sound like the noise was coming from the differential even though it was actually originating in the wheel bearing.
As mentioned earlier, I did find evidence of wear in both the differential and intermediate shaft bearing, so I don't regret rebuilding them. However, it is rather aggravating to know that I could have fixed the noise in a tiny fraction of the time I actually spent on this had I replaced the wheel bearing at the beginning.
Bottom line: If you experience this noise, start by putting the TB up on jack stands, putting it in 4WD with traction control off, and blocking one front wheel then the other front wheel to determine which side of the front drivetrain is causing the noise. Then try replacing that wheel bearing, even if it doesn't sound like the wheel bearing is the problem. If that doesn't work, try replacing the half shaft. Rebuilding the differential is enough work/cost that it is not worth doing until you are absolutely sure that the wheel bearing and half shaft are not the problem.
At least you now have an almost new rebuilt front drive train. Good write up on how to get this "awful job" done (ie. the front differential).