Rod knock?

silver007

Original poster
Member
Sep 2, 2018
16
GA
So I've got this noise. It's most pronounced where I'm at in the video - rear of engine/front of trans. I was really hoping it was a cracked flexplate but it looks perfect. It sounds so "tinny" I thought for sure that's what it was, but I just don't see any signs of plate problems. It does it with the trans unbolted and slid back, although not as pronounced.

Some more details:
The noise comes as you quickly rev, or if you hold the engine in sort of a light load.
The truck was 3 quarts low on oil when it started doing this, around 5k after oil change.
Otherwise, it runs perfectly smooth and powerful.

Should I check for lash adjusters or anything else? Or is this loudness at the bottom most surely a bearing? How can I verify?

2007 4.2 2wd
147k miles

Knocking noise
 
Last edited:

Mooseman

Moderator
Dec 4, 2011
25,262
Ottawa, ON
It sure doesn't sound good. Valve lash isn't adjustable although a stuck lifter is possible but you would hear it on top.

This video shows how to check for rod knock. If you're not sure which one, check them all.

 

mrrsm

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Supporting Donor
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Oct 22, 2015
7,642
Tampa Bay Area
JMO... But THAT sounds more like "Capote's Nightmare"... A Cracked Flex-Plate ... as the sound is VERY EXTERNAL, echoing with a "Trailing Tinny Noise" inside of the Bell Housing with changes in the sound volume and frequency INCREASING when raising the RPM and then DECREASING when lowering the RPM versus what it would sound like if it were being caused by any loose components of the Internal Rotating Assembly. Ordinarily, Rod Knock Sounds DIMINISH with increases in RPM. Here is a similar sounding Video of a 2003 Trailblazer with a Cracked Flex Plate at Idle:


As strange as this might sound... You should also consider checking both Motor Mounts for any signs of the cushioning Gel-Filled MM Bulbs splitting along the carcass walls and allowing that Black Sticky Goo to Ooze and Spray Out onto the adjacent Wheel Wells... as having Bad Motor Mounts collapsing under the weight of the bouncing, jouncing GM Atlas 4.2L engine mated up to the 4L60E Transmission is one of the suspect causes for Cracking Flex-Plates:

LEAKINGMOTORMOUNT.jpg
 
Last edited:

silver007

Original poster
Member
Sep 2, 2018
16
GA
Thanks for this info. I will look at the flexplate again although I've eyed it real good and I don't see any cracks. My motor mounts are about a year old, oem.
 

silver007

Original poster
Member
Sep 2, 2018
16
GA
Well the flexplate has no signs of cracks or wobble. That test where you push down on the cylinders didn't show any movement but the back three are hard to get to so I'm not sure how good I did that. I guess it's time to pull the engine.
 

Reprise

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Jul 22, 2015
2,724
I just watched / listened to the OP's posted video.
I'm going with a bearing (rod over crank, FWIW, although neither is good).
The info that you were 3/7 qts low does factor into my guess, as well.
 

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