mounting a muffler on a soft mounted engine?
Seems to me that if the engine moves and the muff doesn't, you will have problems. How long is the tube between them? I would maybe make that tube longer and give it room to flex. Use something like Macs silicone pipe tubing or something like that so it will flex. I don't think loosening the muffler will help if it is connected to the plane, the air currents will make it move and cause all kinds of problems.