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If GM knows what the problem is and it is as simple as the fan clutch (what they told me) or the belt/tensioner you would think that first the diagnosis would be consistent and that a repair would be available.
did your tech at your gmc dealer tell you fan clutch?
or was there an official tsb or repair bulletin from gm?

I couldn't believe how sound travels through that engine and bell housing. Using a modified stethoscope on mine, I was dead certain that the sound on mine was in the area of the egr components (they make crazy end-of-world sounds through the stethoscope anyhow).

I was wrong.

temporarily removing the aux belt, per gmc instructions posted a few entries up on this thread, proved to me that the thumping sound on my truck is belt, tensioner, or both.

fan clutch is mighty close to aux belt tensioner/aux belt on these engines

did you try/have dealer try the aux belt temporary removal test?



Thanks
 

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Discussion Starter #102
This is the Work Order from November 2019 shortly after I purchased my truck where they clearly indicate that the removal of the belts did not eliminate the thumping/knocking noise. The truck was back to the Dealership in February at which time they tore the engine down and reportedly adjusted the valves( the original comment from the service advisor was that they we’re working on the injectors) after which they advised me that is was the fan clutch and that GM was working on a repair. After numerous visits to the Dealership and absolutely no response from GM I’m contemplating selling the truck to cut my losses. 154D4E28-3A48-4565-AA27-F102E5A44D76.jpg
 

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This is the Work Order from November 2019 shortly after I purchased my truck where they clearly indicate that the removal of the belts did not eliminate the thumping/knocking noise. The truck was back to the Dealership in February at which time they tore the engine down and reportedly adjusted the valves( the original comment from the service advisor was that they we’re working on the injectors) after which they advised me that is was the fan clutch and that GM was working on a repair. After numerous visits to the Dealership and absolutely no response from GM I’m contemplating selling the truck to cut my losses. View attachment 625866
that is awful; can you post a short video of your truck at idle, demonstrating the noise? with my truck, the sound is prominent from the driver's side front wheel well (standing outside of truck, just forward of drivers door). there is no great reason why the aux belt thumping sound shows up there, but it does.

I bought a stethoscope from amazon, removed the "microphone end" and inserted a 20" piece of 1/4" aluminum tubing into the rubber tubing. One of the places that I listened for abnormal sound with that stethoscope was on the fixed bracket between the fan drive belt and the fan clutch, the part that does not move. On my truck, no sound from that area thru stethoscope
 

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Discussion Starter #104
I posted a video of the sound previously it is located early on in this thread, I agree that the sound is louder in the passenger side Wheel well and inside the cab also. Just can’t understand why GM is dancing around on this very frustrating. I will post a new video with the current level of the knocking. I want to take it on a cold start as the knocking is quite loud briefly then reappears when the engine is a operating temp, the truck current has 13,000 km
 

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Discussion Starter #108
I’m at the point where I think I will try another Dealership will let you know how that goes (maybe Ford)
 

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I’m at the point where I think I will try another Dealership will let you know how that goes (maybe Ford)
do you have a neighbor or an employee who can pull that belt?
I have next to zero mechanical skills and I did it on my truck
it takes only a few minutes
 

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Discussion Starter #110
At long last GM contacted me yesterday and advised that they are working on a repair for the throbbing/knocking sound a lot of 2020 Duramax owners are currently experiencing. They advised that the Dealership will be in touch with me when they have a repair bulletin. I asked them if they could provide an estimated date for the bulletin and reminded them that I have been waiting since November, they simply repeated, that I will be contacted when a repair bulletin is available.

Stay safe and look after each other during this unprecedented time of uncertainty!
 

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Count me as another new owner of a thumping 2020 Duramax, with about 650 miles total. Shortly after noticing the thumping at idle, I Googled it and discovered this forum. I listened to recordings posted here and they sound identical to mine. I was was relieved to learn that GM claims there’s no short or long term effects on engine, disappointed there’s no fix yet to eliminate the thumping. Tonight after warming up the engine I did the belt removal check and sure enough, the thumping sound disappeared. Reinstalled the belt and the sound returned. Belt looks good and has no discernible seams, lumps or other defects. Pulleys look good too, and the grooves are clean. Tensioner and fan clutch don’t feel abnormal. The tensioner has a smaller pulley so I would think the frequency of the noise would be higher than it is if it was the culprit, and the thump does seem to occur once per engine revolution. So I don’t think it’s coming from the tensioner pulley.

I did notice there is wiring going to the fan clutch, probably to lock it up when extra cooling is needed. What if whatever controls that is sending a pulse to it every revolution? I’m curious whether pulling that plug would eliminate the noise. Has anyone already tried this? Post up if you do try it please. If no one else tries this by tomorrow night then I’ll give it a try.

I’m just brainstorming here and am pretty baffled so far. But I’m determined to figure it out.

So who will solve this puzzle first...GM or us? Please keep us updated if you discover anything new, and thanks for posting this info.

Even with this annoying thump I still love my new truck! It’s a real pleasure to drive and to tow with.
 

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Discussion Starter #113 (Edited by Moderator)
So who will solve this puzzle first...GM or us? Please keep us updated if you discover anything new, and thanks for posting this info.

Even with this annoying thump I still love my new truck! It’s a real pleasure to drive and to tow with.
Sorry to hear you have the same problem as many of us that purchased a 2020 GM, Duramax equipped truck. My first concern is that I brought this issue to GM’s attention in November 2019, obviously they down played the issue with bulletins as presented numerous times in this thread but continued to sell the trucks with the defect to new customers. I love that you are looking for a solution to solve the problem, but wouldn’t you think it’s a fair expectation considering the price we all paid for these new trucks that GM would want to resolve this?

Hope all of you and your families are safe and healthy!
 

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So who will solve this puzzle first...GM or us? Please keep us updated if you discover anything new, and thanks for posting this info.

Even with this annoying thump I still love my new truck! It’s a real pleasure to drive and to tow with.
Sorry to hear you have the same problem as many of us that purchased a 2020 GM, Duramax equipped truck. My first concern is that I brought this issue to GM’s attention in November 2019, obviously they down played the issue with bulletins as presented numerous times in this thread but continued to sell the trucks with the defect to new customers. I love that you are looking for a solution to solve the problem, but wouldn’t you think it’s a fair expectation considering the price we all paid for these new trucks that GM would want to resolve this?

Hope all of you and your families are safe and healthy!
Yep! That’s a fair expectation for sure. I hope they’re serious about trying to figure it out.
 

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This is unreal,I traded in a 19 high country because of injectior problems and now i hear the thumping you all are experiencing at 200 miles lol... Oh well
 

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I bought my wife's 2017 Tahoe with 24K miles on it and it has made this sound since the day I brought it home. I've always been curious about it.
 

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Add me to that dreaded knocking sound on my 2020 GMC3500. It just started yesterday at about 600 miles.
 

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This sound is absolutely terrible. It’s annoying, loud, and it’s unbelievable that these trucks left the factory with this issue. I spend a lot of time outside the truck while it’s running hooking up trailers, hooking up chains, digging to get unstuck Lol, and this sound drives me bonkers.
 

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This sound is absolutely terrible. It’s annoying, loud, and it’s unbelievable that these trucks left the factory with this issue. I spend a lot of time outside the truck while it’s running hooking up trailers, hooking up chains, digging to get unstuck Lol, and this sound drives me bonkers.
 
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