Friend's 92 400E (W124), 79kmi, has a high-pitched squeal, not very loud, when rolling along slowly; 'sounds like it's from right, rear. Squeal does not change with changes in engine speed, nor when brakes are aplied. Seems to be at wheel, axle, driveshaft rotation rate. Squeal goes away as speed increases, then comes back as lower pitch - not so much a squeal, but as a whine. Any ideas, tips? TIA

Sounds familiar...it could be a lot of things but I put my money on either the rear shock, sway bar bushings/links or one of the arms/links in the rear. There was a big discussion on this topic not too long ago. As a matter of fact, I just changed the rear shocks (Bilstein HD) and one of the links in the rear (based on recommendations from this forum). The bushing in my link were gone!! The difference is night and day. Do a search on "floating 300e' or "rear links/suspension". Not too difficult for a DIY. Good luck.

It's a rotational-sounding squeal.

I'm thinking the universal joint on the driveshaft, but does not seem likely at 79k miles. I would check y
the wheel bearings for roughness and the brake rotors. I had a warped rotor once that sounded like a bird chirping at low speeds. The noise would go away and come back after a sharp turn.
Hope this helps.

Also I forgot to mention the center bearing on the driveshaft should be checked.