please help with rear-end noise

[JHZR2]

Hi,

car is a 91 BMW 318i, 5speed manual, 4.10 LSD diff, RWD.

The car uses 75w-90 redline fluid, which is the weight specced for it.

I get a pretty annoying whine, seemingly coming from the rear end of the car. What I know is this:

-tends to not occur until car is hot from running (5-10 mins)

-tends to only be noticable above 35-40 MPH in 4th and 5th gear (may be because engine noise muffles it at lower speeds in lower gears).

-throttle dependent: seems to be the worst at throttle that is just enough to maintain constant speed. It I let up on the throttle (while in gear) the noise goes away. It also quiets when I press harder on the accelerator. If I let up on the throttle just enough to barely decelerate, the noise goes 'on and off' I suppose due to slight variations in actual throttle pressure and road flatness.

-No noise when I let up off the throttle or take it out of gear.

-turning doesnt seem to quiet it much, although it has been hard to test it on the road at 35-45 mph, as I cant steer the car all that hard without veering off the road.

-differential gets pretty hot after highway driving (maybe this is normal?)

-all fluid levels are correct

-had diff rebuilt a while back because fluid was brown and murky. Quieted noise somewhat, but its still there.

So what do you think? Bad diff? Bad rear wheel bearings? Bad transmission?

Any help wouldbe most appreciated.

Thanks very much!

JMH