Thumping noise with eng running and in park

[jaguarjoe]

It's happened twice now- sitting in a parking lot, engine idling, trans in park, nobody touching anything at all then, out of the blue we hear a thump, exactly like the sound a shopping cart makes when it hits your car. Unfortunately, there is no cart. Near as I can tell, it's coming from underneath, maybe the trans or the TC.
Anybody have any ideas?

 

[CaptainXL]

It's happened twice now- sitting in a parking lot, engine idling, trans in park, nobody touching anything at all then, out of the blue we hear a thump, exactly like the sound a shopping cart makes when it hits your car. Unfortunately, there is no cart. Near as I can tell, it's coming from underneath, maybe the trans or the TC.
Anybody have any ideas?

Hard to say without hearing it. Can you get some video of it happening? You are describing it as a common occurance so that should be no big deal.

Also, do you feel as if you are being bumped from the rear like someone lightly tapped the rear of the truck?

 

[jeffro312]

This is a long shot

If you feel this lump when you hear it and have rear air suspension.

On my Rainier sometimes when bags will be deflated and go to fill up during inflation when i start the car the rear will sort of pop up a little giving a thump noise and a small bump feeling in rear.

Hope I could help if not maybe the next guy

 

[Denali n DOO]

I'll be watching this thread, this happened to me yesterday! I had started up the Envoy, air compressor did it's normal thing, I sat in park because it was cold out and wanted to warm the motor up a bit before I left. After a minute or so a deep big bang thud sound that kinda came from passanger side floor area. I went out a few times yesterday but it only happened the one time. I'll see what happens today it's first run was fine. Let me know if you determine what it was I'm curios now.

It's not the airbag bump from behind sound, I know that sound quite well, mine has new shaft.

 

[jeffro312]

It's not the airbag bump from behind sound, I know that sound quite well, mine has new shaft.

So the air bags filling up bump isn't just the airbags lifting up in suddenly. Increments something to do with a drive shaft or are you just stating you have a new one

 

[The_Roadie]

If the OP filled out his profile, we could know (if it was a trailblazer) if it could have air suspension or not.

I've seen issues with anti-sway bar and links getting hung up on the rear control arms, and if that's the case, you'll see evidence of fresh scraped paint on the control arms. I'd go down there and do a very thorough visual inspection. SuspensionMaxx end links have the most incidences of scrapage (and in one or two cases - outright control arm breakage/bending) in discussions here and on ORTB.

Could also be a bent panhard bar scraping past the differential cover.

 

[Denali n DOO]

So the air bags filling up bump isn't just the airbags lifting up in suddenly. Increments something to do with a drive shaft or are you just stating you have a new one

No I'm just saying it isn't the feeling of being bumped from behind, I had that issue solved by replacing the output shaft, this is different and seemed to come from the floor area. I'm gonna get mine in a garage and check the control arms/suspension stuff like roadie is suggesting. I'll report back if I find anything. It's funny that 2 of us have had this unusal noise that seems similiar.

 

[MAY03LT]

After a minute or so a deep big bang thud sound that kinda came from passanger side floor area. I went out a few times yesterday but it only happened the one time.

The same thing happens in my 03. In park, not touching anything. Def sounds like the pass floor area. I haven't been able to make it do it on demand so there's not much I can do about it.

One time, it made the same noise while driving. As soon as it did it, there was a difference in air volume from the vents. I thought that maybe there was something going on with the recirc door. I did a couple of tests but everything checked out ok.

Are you guys noticing any differences in the air volume when the noise happens?

 

[jaguarjoe]

Roadie- sorry I didn't complete the profile, it is done now though. I dunno how I missed that.

There is no movement of the truck at all, feels exactly like a runaway shopping cart bumped it. Just idling in park in 2wd,no hands, no feet on anything.

It would be hard to record any sound or vision because it has only happened twice in 1 month, totally random.

I doubt if it has air suspension. We never heard it or raise the truck. I'll look underneath today

 

[Denali n DOO]

The same thing happens in my 03. In park, not touching anything. Def sounds like the pass floor area. I haven't been able to make it do it on demand so there's not much I can do about it.

One time, it made the same noise while driving. As soon as it did it, there was a difference in air volume from the vents. I thought that maybe there was something going on with the recirc door. I did a couple of tests but everything checked out ok.

Are you guys noticing any differences in the air volume when the noise happens?

I did think of a door in the ac ducting kinda gettin unstuck or what not but I dismissed that thinking it wouldn't be so loud. After I heard the noise I know I was looking as the passanger floor area where the plenum roughly is. I'm gonna try play with vents n stuff n see if I can get it to do it again. Anyone else have rear floor heat going as well?

 

[The_Roadie]

No sweat, except you have 4WD, not the full-time AWD some other vehicles have.

So you don't have air suspension. Hmmmmmm. Do you have any dents in the bottom of the gas tank? Dented skid shield (if you have that option) under the tank? About all I can think of is a dent that gets pushed out and bangs like a drum when the fuel tank gets pressurized for a vent solenoid test. Have somebody hit various things like the muffler, fuel tank, and resonator with a broom stick, and other parts of the undercarriage, while you sit inside and see if it sounds like you remember.

 

[CaptainXL]

Just a shot in the dark here. But if your water pump was worn or loose somehow the fan could hit the plastic shroud and it would probably sound like a baseball bat hitting a plastic mud flap or something. Just something to check out considering your vehicles age.

 

[jaguarjoe]

No sweat, except you have 4WD, not the full-time AWD some other vehicles have.

So you don't have air suspension. Hmmmmmm. Do you have any dents in the bottom of the gas tank? Dented skid shield (if you have that option) under the tank? About all I can think of is a dent that gets pushed out and bangs like a drum when the fuel tank gets pressurized for a vent solenoid test. Have somebody hit various things like the muffler, fuel tank, and resonator with a broom stick, and other parts of the undercarriage, while you sit inside and see if it sounds like you remember.

I'll put it on a lift Monday or Tuesday and see what I can see. It's got an annoying squeek from the L/R corner, maybe it's the anti-sway bar link.

 

[RayVoy]

One more shot in the dark, (and I can't go out and check, I no longer have the '05) is that the location of the muffler? If the exhaust is coming up to operating temps, maybe something is expanding and moving, maybe inside the muffler.