ORIGINAL: delman
Try tightening the muffler bolts right after shutting off the engine while hot. Also check the balance of the prop to try to remove the excess viration.
Thanks for the reply Delman. The engine is not vibrating excessively, I'd put it in the normal range. When tightening the bolts while the engine/muffler manifoldis hot I tend to strip mout the threading in the manifold, without much pressure/torque applied.