Easiest way is to undo the two bolts that fasten the handbrake assembly to the chassis. There should be two bobbin shaped spacers. Reduce the thickness of these. Two minutes in a lathe or 20 with a file and hacksaw. Usually fixes it. My Saloon rattles a little bit sometimes. I put up with it because I know what it is. The other way is to undo the engine mounting bolts and lever the crankcase as far as it will go in the other direction. Just as effective, not as permanent.
Alan Fairless