With all my local trusted shops booked out to eternity, I'm hoping some of you fine people can assist in diagnosing…
09 H3 CE, 190K miles, all fluids/filters fresh (less than 5k miles) — on the way home from CO there was a sudden, marked change in drivetrain noise*. Most noticeable 30mph and above when throttle's closed and coasting (but still audible, under throttle and road noise), best way to describe it would be somewhere between whining and whirring, tonally close to 100–170Hz, with pitch changing relative to speed, and no perceived oscillation. No clunking, crunching, gear slap, or grinding. No noticeable change in volume, pitch, etc. when turning or load shifts L-R; so I think I can rule out a hub and/or CV issue. This leads me to the front diff. As far as I know, it's original and untouched. I have dropped the gear oil twice (the most recent being 5k miles ago), and removed the cover to inspect the guts when I first got the truck (have not had time to do so again…yet).

Does this sound, even vaguely, like a problem with the diff? If it does, when I take the cover off, is there anything specific I should look for that doesn't require dropping the whole housing? If not the diff, any best-guesses? Btw, I did, at first, wonder about the t-case (rebuilt < 15k miles ago), but the noise doesn't change when 4Hi (L) is engaged. Obviously it would be a little hard to get above 30 in 4Lo, so that remains untested.

Thanks, I appreciate the help!

*Sudden meaning: normal drivetrain noises the first 1,000 miles, then after 50-75 miles off-road (backcountry trails, mild overland—didn't touch lockers or t-case), back on the pavement and this is what I hear.

 

Jmho
I personally would start by removing my front prop shaft and rebuilding it. Takes a few hours and parts are reasonable. Next I'd remove my unitized hubs and front axle shafts and inspect thoroughly [personally id replace hubs and rebuild or replace axle shafts]. Then id replace ujoints in rear prop shaft. Oh and id check fully to make sure i didnt have mud or junk stuck anywhere. Just simple stuff id do before rippinf apart my diff. Of course your t case could be having trouble again.

 

Nothing caught in the drive train from the off-road? Sticks, etc.? Mud in the brakes, maybe caught in the parking brake drum?

Front drive shaft is easy to check.

 

Buy a front diff gasket and prepare to pull the cover for an inspection. Drain the front diff and look for glitter (metal). If you are 100% absolutely sure there is none, you can leave the front cover on and refill. Since you have not looked in there for a long while, might want to do so anyway while it is drained. Do a Google on what to be looking for besides the obvious worn gear teeth, chipped tooth, etc...

 

Thank you everyone for the suggestions—I appreciate it and will be getting it back up on the lift this weekend to follow your leads…

HH: Thank you for the thorough run down. I'll be inspecting the prop shafts and hubs (I wouldn't have considered the prop shaft)
brothapig: Had that thought; went under and inspected, hoping for foreign objects or debris. All clear.
Doc: Appreciate your input as always. Gasket is ordered. Whether or not I find glitter, I've got a Blackstone Labs bottle here somewhere and will be capturing some of the gear lube to send off for analysis…

I'll update soon—thanks again!