I'm getting some pretty significant grinding sounds whenever my drivetrain goes slack (i.e. when I roll off the gas on a downhill, or roll onto the gas after coasting). After I've rolled off the gas and the drivetrain is reverse-loaded coasting downhill its fine again. Whenever there's load it's totally fine, but sounds terrible when slack, even with the clutch in, and no matter what gear it's in. Any advice on what this might be? T case? Transmission? Center dif?

 

My 2004 did this shortly before the front diff exploded on the highway. Drain your diff fluid and see what it looks like. If there's glitter, you found your issue.

 

Thanks! Will check this shortly. Did you find it only happened when the drivetrain was slack? The grinding I'm getting really feels like gears not quite meshing.

 

It got worse quickly over time, but initially the noise occurred when driving on the highway and letting off the gas. And also when coming down an off ramp to a stop.

 

I see... I think my case might be a little different, it only happens momentarily when no load on the drivetrain (like when you roll off the gas), as soon as the drivetrain is loaded again (like if coasting downhill in gear), it goes away. I'm suspicious of transfer case.

Thanks for your help!

 

Same here. Basically any time the gas was let off.

I mean, you’ll have your answer in five minutes if you just go drain the diff and look for metal.

 

Well I pulled the dif plug the oil was nice and clear... Which is bad news for the the transfer case I suspect.

 

Driveshafts okay?

 

Hmmm, crazy. I definitely would've put money on the diff. Welp, I guess do the same to the TC and see. Worst case with all this is your diffs and TC get a fluid change.

 

Update:
The grinding has gone away on it's own, but now there is a pronounced whine (not belt-like) when decelerating by engine braking. It is quite pronounced and only occurs when decelerating (i.e. when the drivetrain is reverse-loaded).
I've now drained the transfer case and found quite a bit of shavings on the plug and in the oil. Very fine shavings, more like glitter/shimmery stuff, no noticeable lumps that I could really see or feel with my finger tips.
After replacing the TC fluids (it was low by about half a quart) with royal purple 75w140, I still get the noise when decelerating.
Next up I guess I'll disconnect front driveshaft and drive around with CDL locked to rule that out. I'm also planning on draining and filling the trans fluid (with Redline MTL).