my 87 TD has a rolling rumble...
 

I looked up under the car, and found the differential coated in a tad bit of gear oil...
I'm now putting the car up on ramps, and going to pull the fill plug, then top off the gear oil... I hope that quiets it.

... 124 fill plug is not the same size as the 123/126... it's smaller. my 14mm socket will not fit.
is it 13?

dunno... seems like I used the same for all three(123/24/26). Did you spray the plug out with brake cleaner? I only have one extra large allen wrench that I use for diffs and I have drained from all 3 types.

My '87 diff has a nice rumble.. goodluck

I've got a few spares, but I REALLY don't wanna damage this one.
I tried cleaning it and cleaning it. no go.
it would NOT fit. I'll dig in there, and scrape and clean it. tomorrow after work... I hope it makes it...

... diff is full. and clear fluid.
left rear tire had split and was at the point of showing tread. put new tire on... no change in noise.
I'm thinking it's either a wheel bearing, or center support bearing...
it's all next on my list...

oh. and a new 14mm socket fit like a glove.
no idea what was up with the allen wrench in my kit...

Fit like a glove on O.J. Simpson? That might be your problem.

If it changes with throttle it is more likely the shaft bearing, if not then try swerving gently, if that changes it then a wheel bearing.

X2

Also, how much back lash (play) do you have in the diff ? Try turning the pinion.
While I have never heard of it on a MB, bad spider gears in a diff can make a rumble.

My loud rear diff is very sloppy... I thought it was normal until I got a replacement and saw how tight it was.

it's hard to describe.
the sound is NOT rpm dependent. totally speed.
I can get no change in pitch from accelerating/decelerating... no change from cornering. I HATE to tear the car down to measure play in all the candidates... ugh.

How many miles are on it? When you get under it again grab the driveshaft near the center bearing and see how much play it in it.

This sounds exactly like what I hear in my '84 W126 300SD... it's speed dependent only, engine revs not involved.

I had an experienced indy MB shop owner drive the car who said it's the front pinion bearing in the differential. He said they'll run this way for a long while if the diff. lube is kept up to proper level and the wear/noise is not really excessive.

Since mine is not too bad I've not felt it necessary to pull the axles, the ring gear, the driveshaft, etc. to replace the bearing. Obviously, if the bearing is worn, its race should be replaced also, which is why the ring gear must come out.

Just keep the gear oil up to proper level and change it at least once a year to get rid of loose metal, however small the particles.

My 300TD has the same thing...rumble, only related to wheel speed, increases in pitch with higher road speed, doesn't change when I swerve, no change in volume when I coast in neutral. I figure it's gotta be a diff bearing or the center bearing. I realized I really wasn't prepared to change any of those myself right now, so I'm handing it over to the shop for diagnosis. They can no doubt put it up on a lift and run it to find the noise, something I can't safely do at home. It's quite loud and annoying at highway speed.

since my noise is exactly rotational with wheel speed, I think it's either rear wheel bearing, or axle bearing in the diff.
there is fluid on the passenger's side of diff. on the half shaft, and on the pumpkin.
likely the diff axle bearing is failing, and it's taken out the seal on the diff... allowing oil to seep out.
I'll let it go until I have time to swap diff/axles/subframe parts from my 95TE...

I shoved a 1/2 inch diameter bolt in mine and welded it in place. It now comes loose easily with an 11/16 wrench.