I have a similar problem with my 1990 525i. Sometimes while driving a strange groaning/whirring sound comes from the dash seemingly right behind the radio. The sound varies with engine RPM and usually stops after a second or two after I let off the throttle (or maybe it really only lasts that long and I just do that instinctively). I can't find any pattern with it, seems to happen at completely random times. It's quite infrequent and short lived so I've just been ignoring it.
Transmission: 5 speed