2013 and 700 miles!!!! Has it been waiting for repairs that much? I would check the belt tension and maybe the bevel gearbox oil, with that mileage and age you may have "the gunge" in the BB rather than oil. Agree with Michael H about the sprocket wear, although my cars sprockets lasted at least 2000 miles before starting to complain. There is a steel sprocket.
I selected my username a bit prematurely before I became an owner - My M3W was built in August, 2014.
I changed my bevel box oil about a month ago and the BB is reasonably quiet. This noise is different from the bevel box whine, which I think I've trained myself to ignore because I rarely notice it anymore.
Also, the belt seems to be fairly tight already. I tried fiddling with it and couldn't twist more than 1/8" either way.
