If you have a M3W with the NVH kit on (post 2014- the big steel beam between the bevel box and the gearbox) the rubber mountings can allow the bevel box to move under acceleration and deceleration. It nods and the front input flange can hit the chassis, there will probably be some witness marks. Some cars are more prone to this than others and the drive belt has to be a bit tighter on NVH equipped cars. Phil Bleazey has produced a BB mount that fixes the flexing in the BB mount. Info here:
http://www.bleazey.co.uk/M3WBBinstallationmanual.html. I have this fitted to my car and it works well.
