Having fun driving my truck now that I've got the motor tuned pretty well, but I've had a pretty bad noise coming out of my transmission since the beginning. It's a C6 behind a 400, connected to a (divorced) NP205, 9" rear, and D44 front (driveshaft and pinion in place in the front, but no carrier or axles yet).

Here's how it goes: when driving below about 15 mph (don't know for sure... speedo doesn't work either...), there's no problem. But when you get up to at least approximately 25 or 30 mph, it makes a terrible grinding sound when you back off the throttle. It sounds exactly like someone trying to force a manual transmission into the wrong gear.

Let's say you accelerate from a light up to 35 mph at 45% throttle. Well, when you hit the speed you want to cruise at, you back off to maybe 25% throttle. It's at that point that the grinding starts... right where the drive train is sort of floating between "drive" and "coast". If you apply a little throttle, it'll go away. Or, if you completely let off the throttle, it'll go away then too.

I would be inclined to say it was a transfer case problem, but it happens in 4-hi and 2-hi, makes no difference. Also, since my xfer case is divorced, it sits quite a ways back, and the sound seems to be coming more from the tailshaft area of the trans.

The transmission actually shifts pretty well, a little soft from 2 to 3, but other than that, no slipping, shuddering, etc. Shift points are fine, modulator seems to be working. No leaks either. U-joints on the rear shaft and intermediate shaft (between trans and xfer case) are brand new.

Anybody have any thoughts on this? Point me in the right direction?