2020 l5p duramax knock at idle cold, worse hot


Jun 18, 2016
Farmington, MN
Was just helping a buddy on his cabin over the weekend and his ‘20 L5P makes the same noise.

That would be great if it’s an airbox issue, that would be an easy fix, but it sure doesn’t sound like it to me.

If you guys look at post #22 by @2004LB7 He posted a link to a tech document on it. That document basically states that once the belt is removed the noise seems to go away and it is “normal operating sound”. Which is bologna.

I‘ve got an LLY I’m working on right now with 310,000 miles and it sounds so much better and refined than the L5P.


don't know sh!t about IFS
Staff member
Oct 21, 2009
Phoenix Az
wonder if its a combo of the typical "dmax crackle" you hear on a fix vane charger (all dmax trucks have this, just dont usually hear it on a variable vane) and the fact the cat is right at the back of the turbo creating a megaphone when the turbo goes open vanes. seems the vanes are open more at idle on these trucks than the older ones? i have to REALLY listen to my truck to hear it and it only happens every once and a great while so its hard for me to investigate more.

it would seem viable since the L5P if the only year where they moved the turbo forward and put the cat right at the back of the turbo.

wonder if deleted trucks do this?


New member
Jan 30, 2021
I bought my 2021 Duramax in November. The dealership also told me that the noise is character of the engine. I can’t remember what the mileage was when I first noticed the thumping noise, but I took it to the dealership and they performed the service for the bulletin on it. Removing the belt and cooling tube and running the engine FOR NO LONGER THAN 3 MINUTES! Did they follow the instructions? I don’t know but when I picked it up and drove down the highway, it has a driveline vibration that definitely wasn’t there before! It worries me to think that I paid so much for something that I might not get the mileage out of before it needs major repair!


New member
Jun 30, 2015
here’s a quick clip of the thumping. It sounds like the little drummer boy is under there. . . . .

I am currently working on an 18’ 3500 WT with that same noise, have you found anything out? I performed a head gasket repair and the owner swears the truck wasn’t that loud before the incident happen. I have no codes and injectors were check out and passenger side head was replace to due to #3 area of head being domed by .008 from the rest of the deck. The cylinder wall looked good and piston wasn’t clean; soot/carbon color look liked the rest of the other 7 pistons, and un damage. Using conventional rotella 15w40.

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