At high speeds I'm hearing a high, whistley kind of a whine coming from the rear end of my 240DL. It sounds sort of like the sound of a jet engine, only much quieter. It starts at around 50 mph and gets higher in pitch at higher speeds. The sound seems related to ground speed only and happens in and out of gear and with the clutch in or out. It makes this sound during gradual highway curves as well as straight ahead driving. Unfortunately it's difficult to really hear exactly where the sound is coming from, other than the fact that it's coming from the rear of the vehicle.

Judging from my desciption of the sound, would this be a bearing problem or maybe something in the differential? What would I do to diagnose it further or fix it? Should I wait until it gets so bad that it's more obvious what's wrong?