2004 Chevy 2500HD, 6.L, auto, 4WD, 52K miles.

I'm getting this awful grinding/ratcheting sound when decellerating (coasting), usually downhill, usually around 45-50 MPH. When this happens, the RPMs drop to 1K, and the noise slows down as the truck slow down. If I just tap the gas, the noise stops and the RPMs return to normal. The truck seems to drive fine otherwise. I can drive the truck for 100's of miles and this will not occur. Other times it will happen a few times in a short distance. This is what's making this tough to pinpoint. I searched al over the internet, stopped in a few transmission shops and talked about this but no solution. I was reading in the links below that it could be a transfer case issue. Back snap ring?

Dave, did you solve the issue with your truck? Mine is doing same thing.

I ended up replacing the transfer case. Cost me around $1800, :-(

I had similar situation, I changed the front and rear u joints and problem solved

Thanks for quick reply Dave, if there is a silver lining didn't have to do the tranny.
cigrocket already tried u-joints, thanks for input though.

Copy. I was hoping it was the cheaper of the 2 for you. Funny, that sounded exactly like my problem. The transfer case sounds logical also. Good luck, let us know how you make out.

Mine started out being very intermittent. I could drive 100's of miles and no noise. Eventually it started making the noise a lot.

Search GM Duramax transfer pump rub kit. Common issue on the Allison transmission. This could be causing the problem.

Your description, right down to the vibration (mine starts around 35 mph thru 55) is so spot on I thought maybe you stole my truck 😂😂. Mine has even started doing the grinding in reverse. I'll try to remember to post the final results.

I had a buddy just go through a similar situation with his 8.1L 2500 Suburban (very high mileage 300k+ so many suspects). He was a mechanic too, drove him nuts. Would only make sound under deceleration after vehicle warmed up. Changed CV joints, rebuilt transfer case, u-joints good, new(er) rear axle, ect. and problem would vanish for few hundred miles and return. Made a hell of a racket like gears grinding.

Come to find out gas tank vent was plugged and tank was swelling enough where periodically heat shield would rub on driveshaft. Took months for it to actually leave noticeable witness marks on the shield. Whenever he would put on hoist to look, tank had shrunk and plenty of clearance was visible. That was a strange one and we both had multiple conversations trying to figure out what the hell it could be, had a good laugh afterwards.