Finally took my M3W to the local dealer. Relatively quickly he diagnosed the problem to be a drive belt that was looser than factory recommendations, so that was rubbing on the small aluminum stone guard just in front of the rear tire.
Some posts on TalkMorgan had recommended adjusting the belt so you can twist the belt 90 degrees....in hopes of protecting the bevel box. I had loosened mine to be able to twist somewhat less than 90 degrees, but still more than the way it came from the factory.
Dealer claims that the bearings, etc., in the bevel box are so robust, that protecting them simply isn't a worry unless you tighten the belt crazy tight. According to him, 10mm flex or movement at the center of the belt is proper.
Problem solved.
