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So glad that I found this post. I am having very similar issues with my 2018 Sierra 2500 HD L5P, only 52k miles. I had both of the smooth pulleys and serpentine belt replaced. Still having the same noise. The tech finally used a scope and seemed to have narrowed down the noise coming from the area behind the crank shaft pulley. Is this the area where your gears were replaced are?
 

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might be the issue?

Harmonic Balancer Sliding Back on Crankshaft
July 22, 2020
A belt chirp sound or metal-to-metal sound may be heard on some 2019-2020 Silverado HD, Sierra, HD and Silverado 4500HD/5500HD/6500HD models equipped with the 6.6L Duramax diesel engine (RPO L5P, L5D).
The outer ring of the harmonic balance may slide back on the crankshaft and cause a misalignment of the accessory belt. If the outer ring slides back far enough, it may contact the timing cover and oil pans.
Inspect the harmonic balancer, timing cover and oil pans for any damage and replace as needed. (Fig. 10)
Fig. 10
Check for damage to the harmonic balancer due to contact. (Fig. 11)
Fig. 11
Also check for separation of the harmonic balancer’s inner and outer rings. (Fig. 12)
Fig. 12
The damper has a hub that is longer than the inertia ring. Look for a witness mark where the elastomer was originally positioned to identify any movement. (Fig. 13)
Fig. 13
Refer to the appropriate Service Information for the proper procedures to remove the harmonic balancer, timing cover and oil pans.
For additional information, refer to #PIP5742.
 

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Thank you for this. I took a look this morning under my truck and I didn't see anything from the exterior of the area but by those pictures this could be occurring behind the harmonic balancer. I have an appointment on Monday to bring it in to the dealer and get it looked at.
 

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I have my truck in at the dealer and they ordered a new harmonic balancer for the truck. Should be put on today/tomorrow morning and will let you all know the outcome.
 

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might be the issue?

Harmonic Balancer Sliding Back on Crankshaft
July 22, 2020
A belt chirp sound or metal-to-metal sound may be heard on some 2019-2020 Silverado HD, Sierra, HD and Silverado 4500HD/5500HD/6500HD models equipped with the 6.6L Duramax diesel engine (RPO L5P, L5D).
The outer ring of the harmonic balance may slide back on the crankshaft and cause a misalignment of the accessory belt. If the outer ring slides back far enough, it may contact the timing cover and oil pans.
Inspect the harmonic balancer, timing cover and oil pans for any damage and replace as needed. (Fig. 10)
Fig. 10
Check for damage to the harmonic balancer due to contact. (Fig. 11)
Fig. 11
Also check for separation of the harmonic balancer’s inner and outer rings. (Fig. 12)
Fig. 12
The damper has a hub that is longer than the inertia ring. Look for a witness mark where the elastomer was originally positioned to identify any movement. (Fig. 13)
Fig. 13
Refer to the appropriate Service Information for the proper procedures to remove the harmonic balancer, timing cover and oil pans.
For additional information, refer to #PIP5742.
Well the dealer replaced the balancer and the sound is just as prevalent as it was before. Perhaps its the oil pump gears as the original poster explained? Nonetheless the service tech says to bring it back next week to dig deeper
 

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Discussion Starter #46
You might have them call Bud Clary Chevrolet in Moses Lake, WA. at 509-765-4517. I believe the service tech's name was Tony. I don't recall the service managers name but he took video of the truck/noise and sent it to Chevy in Detroit. A tech there was the one that had heard of the problem and said it was the gears on the oil pump and the cam. I don't have any of my paperwork handy but they should have a file on my truck - my name is Paul Harder.

On the bright side, I have over 30k miles on the truck now and NO NOISE, great mileage (for a big truck) and comfortable!

Anyway, best of luck on getting your truck fixed!

Paul
 

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Thanks Paul. Did yours make the noise once it was warmed up for a few minutes and then was fairly consistent at idle? Mine is currently at the dealer waiting to be looked at. Sent you a message!
 

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Here is the sound that my truck is making. I am beginning to wonder (after searching part diagrams) that it could be a cooling fan pulley bearing.

 

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Here is the sound that my truck is making. I am beginning to wonder (after searching part diagrams) that it could be a cooling fan pulley bearing.
Did you see this one?
Shows failed belt tensioner,


I
 

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Discussion Starter #51 (Edited)
Well, I'm showing my age......can't get it from my phone to this forum! GRRRR!
 

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Discussion Starter #52
Ok, finally got partially tech savy, LOL!
 
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