Rc e30?

[Scallywagon]

There's a whirring sound, like a gigantic DC motor is powering the rear wheels @ the rear wheels, sound increases with speed, not RPM.

Sounds like tire rubbing on something, but doesn't stop over bumps or when the suspension gets unloaded. Doesn't sound worse with the windows down

I'll be going over the whole suspension back to front as funds permit, however if I can dial this in, it will get first dibbs.

[LivingLegend06]

Sounds like the diff. Start by replacing the fluid with redline synthetic gear oil, if that doesn't work replace the diff.