500e driveline howl

Over 80mph or so I get a howling in the driveline. The sound is there when accelerating or at steady speed but goes away when coasting. It's not too loud and definitely not exhaust, temperature or rpm related (although it sounds like a 600cc sportbike is behind me)

The car has 126k and as far as i can tell from previous owners receipts no trans or driveline work other than fluid changes.

The trans is not unusually clunky, is there a bearing somewhere on it's way out?

Thanks,

Christian J
'92 500E