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There is no way that is a valve, rocker arm, lifter noise. The only time that I heard a similar sound was my 68 GTO when it spun a rod bearing. It is not a rod because the noise would increase while increasing and decreasing RPM from idle. Mine sounds to me like a single stage air compressor running in the back ground in time with the engine RPM. It is driving me nuts but I am skeered to death to have some Ace mechanic tearing things apart on a hope or a maybe or just a simple I duh no!!! It is a more than good chance that the result will be a permanent screw up that, if your lucky, toasts the entire mill before the warranty is over!!!
I agree totally. I watch engine oil pressure, if it was a rod or main bearing the oil pressure at idle would be lower than the 25 psi my engine oil pressure gauge is indicating, This is more like a hollow thumping sound. It is simply annoying. If I had access to a lift which could support the truck I would be under it and narrow down the “thump”. It is the only negative I have about the truck, so much so, that if it was present on day one of the sale, I would have walked !!
 

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I agree totally. I watch engine oil pressure, if it was a rod or main bearing the oil pressure at idle would be lower than the 25 psi my engine oil pressure gauge is indicating, This is more like a hollow thumping sound. It is simply annoying. If I had access to a lift which could support the truck I would be under it and narrow down the “thump”. It is the only negative I have about the truck, so much so, that if it was present on day one of the sale, I would have walked away from the sale!
On a truly sad note, no one at the dealer is diesel certified.
 

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There is info on this at GM Techlink. Seems to be the aux drive belt and the sound is amplified by the bell housing
 

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There is info on this at GM Techlink. Seems to be the aux drive belt and the sound is amplified by the bell housing
This may be true but how does one explain to a customer who spent anywhere from 60-85K on a truck that the sound is normal? GM needs to come up with a replacement belt or drive system to eliminate the thump, thump, thump. I think you will agree.
 

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I do agree. How can GM simply say "Oh that's normal". Why do some of the same model engines not make the noise and some do? It's a simple belt drive system with a single belt.
 

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I do agree. How can GM simply say "Oh that's normal". Why do some of the same model engines not make the noise and some do? It's a simple belt drive system with a single belt.
Only 2020's do it. They have a 2 belt system.

2017-2019 have a much different accessory drive system. Single belt.
 

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From the above link:

Duramax Diesel Warm Engine Idle Knock Sound
April 15, 2020
Some 2020 Silverado 2500HD/3500HD and Sierra 2500HD/3500HD trucks equipped with the 6.6L Duramax diesel engine (RPO L5P) may have a thumping or knocking sound coming from the rear of the engine. The sound is often more pronounced in the interior of the truck as it’s amplified by the transmission bell housing.

To duplicate the engine sound:

Idle the engine at about 600 RPM with the transmission in Park or Neutral. The engine oil temperature should be 113–185°F (45–85°C).
Turn the HVAC fan and radio off.
Remove the accessory drive auxiliary belt. (Fig. 14)
If the noise level is significantly reduced, it should be considered a normal characteristic of the 2020 model year truck. No additional repairs are necessary.

Fig. 14
1076316


Use the following procedure for accessory drive auxiliary belt removal, testing and installation:

Remove the air cleaner housing.
Remove intake air duct.
Separate the charge air cooler (CAC) tube and position aside.
Remove the belt and position aside.
Install the CAC tube.
Loosely place the air box and intake duct back in place. (So a DTC is not set for the sensor when starting.)
Start the engine and verify the sound is gone.
Remove the loosely installed air box and intake duct.
Separate the CAC tube.
Install the belt.
Install the CAC tube.
Install the air box and duct.
TIP: Do not run the vehicle more than three minutes without the auxiliary belt installed. Do not raise the RPM higher than idle with the belt removed.

The engine sound has no short- or long-term effects on the engine.
 

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Totally speculation but I would not be surprised if the harmonic balancer or the pulley bolted to it is the culprit. The only way the noise can be amplified by the bell housing leads me to believe the crankshaft is transferring the “thumping “ sound from the front of the crank to the rear of the crank to the flex plate and torque converter. Hopefully GM is actively working on this. With the pandemic I highly believe anything constructive is going on and I don’t blame them .
 

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Red, I knew it was 2 belts-- meant that it was the single fan belt that is the culprit. Thank you for posting the info and clarifying the belts.
As for why the noise seems to be louder at the rear-- the bell housing is just that-- a big bell.
Watched a Banks video on the new 2020 L5P and supposed reason for additional belt was fan is 2 inches bigger. Wonder why one belt wouldn't drive it?
 

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Red, I knew it was 2 belts-- meant that it was the single fan belt that is the culprit. Thank you for posting the info and clarifying the belts.
As for why the noise seems to be louder at the rear-- the bell housing is just that-- a big bell.
Watched a Banks video on the new 2020 L5P and supposed reason for additional belt was fan is 2 inches bigger. Wonder why one belt wouldn't drive it?
Gale Banks video is great and explains much about the 2020 L5P except he does not address any issues other than broken crankshafts and the install of a Fluid Damper viscous balancer. If it is simply a drive belt issue then simply replacing the belt with another design should eliminate the thumping. Goodyear Gatorback now Continental is the best belt out. There is no listing for the 2020 Duramax
 

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Again I agree. If removing the belt stops the noise why doesn't GM investigate the belt material, idler setup, and lower additional pulley?
If only some trucks have this problem I would lean toward the belts or tensioner pressure on the belt. Maybe the lower pulley is ever so slightly off center?
 

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Did you note the water pump or power steering pump involvement?
Howdy. I only needed to remove the fan belt to eliminate the thump noise, and the fan is the only thing driven by that belt. It’s the front belt and removing it is a breeze. When I reinstalled it the noise returned. The rear serpentine belt is the one that drives the water and power steering pumps. I still haven’t tried my idea of disconnecting the fan clutch wiring plug to see if that eliminates the noise, but that’s probably not the problem anyway. Then again I didn’t think a belt could make this noise, but on my truck, it’s definitely part of the problem. I’m looking forward to trying an aftermarket belt when one becomes available.
 

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I was doing som e underhood stuff on the 4500 and found that it has a seperate belt for the fan as well and it is a 2019 but a L5D. It is a wider belt than the other belt- it that like the 20's?
it has never made any noise like described-- only a faint knocking on decel due to injector controls.
 

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Gm had a quality control guy contact me yesterday in regards to the Thumping sound. He said they are working on the issue, he called because on my new vehicle survey I stated this was an issue.

Hopefully more owners did those, and if you have not try to get it done, they apparently do notice and are serious about this. He gave me his contact info if anything gets worse to a point im mechanically worried about the truck, which im not. He asked if I had it to a dealer and if they took it serious ect..
 

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Gale Banks video is great and explains much about the 2020 L5P except he does not address any issues other than broken crankshafts and the install of a Fluid Damper viscous balancer. If it is simply a drive belt issue then simply replacing the belt with another design should eliminate the thumping. Goodyear Gatorback now Continental is the best belt out. There is no listing for the 2020 Duramax
Tonyg
Try getting one for the 2019 4500HD like mine. It has the 2 belt system and doesn't make any noise at all. Not sure why the info doesn't reference the medium dutys at all-- maybe the owners just haven't complained or the problem doesn't exist on a L5D engine. Same engine for the most part.
 

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Gm had a quality control guy contact me yesterday in regards to the Thumping sound. He said they are working on the issue, he called because on my new vehicle survey I stated this was an issue.

Hopefully more owners did those, and if you have not try to get it done, they apparently do notice and are serious about this. He gave me his contact info if anything gets worse to a point im mechanically worried about the truck, which im not. He asked if I had it to a dealer and if they took it serious ect..
Too bad GM doesn't have a tech guy a member of this forum-- they would probably learn a helluva lot more about their equipment from a lot more knowledgeable people than their dealer departments
 

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Mine is so loud now that i can here it when im 30 or 50 ft from it.. Its really embarrassing when one owns a new truck with the temporary tags on it and it draws attention when i pull in to fuel it up.. Im asked is that engine knocking , it sure sounds like it lol
 

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Tonyg
Try getting one for the 2019 4500HD like mine. It has the 2 belt system and doesn't make any noise at all. Not sure why the info doesn't reference the medium dutys at all-- maybe the owners just haven't complained or the problem doesn't exist on a L5D engine. Same engine for the most part.
How wide is your belt?
 
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