My E38 in the last few days has started to exhibit a very strange vibration when slowing down, almost to a stop. It's a grunting or groaning sensation that is felt and heard, low-pitched, like that sensation when your brakes aren't quite holding the car still. Figured it was the brake pads, and it still does it when I pop the shifter into N and apply the brakes - it stops when I let off the brakes and resumes when I put them on, as long as I'm going about 5-10 mph.

But after driving for at least 15 mins., when I stop and put the car in reverse, before even moving the vibration appears and is worse than ever. It disappears after backing up a 50 foot driveway. I'm really puzzled, because if it were the brakes then being stationary in R would not cause it. Doesn't seem to correlate with turning the steering wheel although due to weather I haven't checked the PS fluid level yet.

Almost has to be driveline... rebuilt driveshaft and new guibo about 10k highway miles ago. I'm worried that my 5HP24 is about to blow up. It drives and shifts perfectly through all forward gears + converter lockup. Backs up OK too, just the weird grunt/groan after putting it in reverse. No errors on the dash display.

Any thoughts as to what this could be? I've never experienced anything like it.