I saw on the other thread there was talk of a rev limiter - you're not hitting that are you?
It's quite a soft rev limiter if/when you hit it and is very unlikely to casue the violent banging.
99.9% the bevel box moving so, as above, first tighten the belt and then maybe consider Phil's upgraded mounting.
As a quick check - you should only be able to twist the lower run of the drive belt about 45 degrees or slightly over.
