My M3W is suffering from exactly the same problem.
i.e. sounds like a knackered old tumble drier and highly embarrassing when you pull up next to pedestrians.
It is def the belt , not the bevel box.
I have done 3.5K miles by the way on the first sprocket.

A little bird has told me a steel replacement is in development , to match the steel sprocket on the bevel box , which doesnt seem to wear. I have tried for a sample but failed , early days was the reply. So in the mean time I am pushing my dealer for an ally consumable one until the 'final solution' arrives.

Last edited by nippymog; 23/07/14 07:27 PM.