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Howdy fellars. I can be added to the thumping sound also. Just over 600 miles on mine.
 

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There is a TSB on this issue and GM is now saying it’s a normal sound of operation. That’s total crap. The typewriter tick is one thing, Barely noticeable, but this is much louder, and much more irritating.
 

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Discussion Starter #124
I am planning on buying a 2020 or 2021, but no way until this is resolved!
I purchased my 2020 2500 Duramax in November 2019, it currently has 15000 km on it. The throbbing sound can now be heard all the time and is especially loud at cold start. Gave up trying to deal with it at the Dealership or with GM I regret my purchase but have few options, no one is going to pay a fair dollar for the truck until this is resolved and this is certainly not an issue that GM is concerned about.
 
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I purchased my 2020 2500 Duramax in November 2019, it currently has 15000 km on it. The throbbing sound can now be heard all the time and is especially loud at cold start. Gave up trying to deal with it at the Dealership or with GM I regret my purchase but have few options, no one is going to pay a fair dollar for the truck until this is resolved and this is certainly not an issue that GM is concerned about.
I just purchased a 2020 GMC Denali 3500 with 54 miles and it has the same sound. I don't think I am going to worry about it at this time. If it gets worse then that will be different.
 

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I am planning on buying a 2020 or 2021, but no way until this is resolved!
If your waiting for it to be resolved or good news, you will never buy. My truck has the same noise. Am i concerned? yes a little but diesels make noises and i have warranty and a good dealer. I agree that its not right but i 100% believe its not at risk to the engine. If I were going to base a purchase or not off a forum on the internet id never buy. most times the good news is never on here its all the ranting and crabbing about issues with trucks. I went from a 2017 to a 2020 and have absolutely no regrets and always figured that there is no way it would get better than my 2017. I was wrong.
 
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Discussion Starter #127
If your waiting for it to be resolved or good news, you will never buy. My truck has the same noise. Am i concerned? yes a little but diesels make noises and i have warranty and a good dealer. I agree that its not right but i 100% believe its not at risk to the engine. If I were going to base a purchase or not off a forum on the internet id never buy. most times the good news is never on here its all the ranting and crabbing about issues with trucks. I went from a 2017 to a 2020 and have absolutely no regrets and always figured that there is no way it would get better than my 2017. I was wrong.
Glad to hear you’re satisfied with your purchase, personally I expected that the problem would be resolved by now I guess my expectations are too high.
 

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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.

634601
Fig. 14

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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Discussion Starter #129

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.

View attachment 634601 Fig. 14

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.

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.

View attachment 634601 Fig. 14

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.

BD1C3370-9840-4ECC-A5F1-2E079359DEBE.jpeg
 

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I am planning on getting a new truck to replace the 05 Duramax. I am thinking it might be an alignment issue. the belt is trying to walk out of the shiv. Watch the belt and see if it tries to climb the shiv or pulley. I used to be a mechanic for Del Monte. We would loose belts and it was because the pulleys were not aligned right. I was thinking that maybe there is a little misalignment. just enough to make the noise.
 

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Just took delivery of mine and only have about 80 miles on it and mine seems to sound the same as well. Goes away by the time I drive down the block.
 

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Genewman,
I just watched the video you posted.
I just bought a 2020 3500HD (1500 miles.)

Mine is making the identical sound, and it seems to get louder under heavier loads.

Any update from GM?
Pretty annoying for a $74k vehicle, right?
 

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2020 Silverado 3500HD Duramax/Allison, LT, Crew Cab, SRW, Std. Bed, Z71, 4WD,
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Add me to the list. Just picked up mine on Monday. Noticed the same sound when we were in the drive thru line at In-N-Out. The GM bulletin says no short or long-term reason to worry, but I will bring it to the dealer's attention anyway. My Brother in law says his 2017 does the same thing. If GM follows their usual process, they won't do anything to correct it for another 5 years.
 

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I've settled it's going to have it at times, not a big deal anymore for me as after 21k of pulling and running hard I have zero mechanical issues. So it is what it is

On a side note my co worker has a brand new 2020 f350 power choke and it has a small thud that my 2018 power choke didnt. He also has their new 10spd tranny.

Not saying there is a relation but it's a noise my Fords didnt previously have and is very similar to my GM

Sent from my SM-T837V using Tapatalk
 

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dam its a new truck, shouldn't have to listen to something making a knock and make you think there is something wrong????
 

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I still keep looking for anything on this from GM, nothing. I did come across something other than the belt removal. I was watching part of the banks videos on YTUBE. The oil cooler is bigger with more cooling coils and its on the back of the engine. I'm not saying that is what it is but it is a new part put on the 20's. I have been looking at what the differences are and what could be making the noise. I started to think that cold oil going thru the cooler may not make the noise till it warms up and makes it till it cools down. Till its cold again and I don't get it when its cold. I will think things thru, that is till I either give up of find the issue. I have emailed a few places with questions and ideas and most of the time I get nothing in return????
 

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or the geometry of the belt path -- multiple short, tight runs and a few runs that in comparison are quite long

One would have hoped that the gm engineers would have modeled this before creating the spec for the tensioner force, allowable belt stretch, etc.

we really need to know if the 2020 dual-alternator duramax models have the thumping sound --> different belt path, different opportunity for resonance
Yes, I have 2020 with dual alternators. Makes the thumping sound. First noticed around 300 miles. Now 800 or so. Soon as I start in the morning it’s there. Seems to go away sometimes. Local dealer says normal
 
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