When you shut it off; use a piece of metal conduit or copper and use it like a stethoscope to isolate the noise. My guess would be the muffler. You could pull the spark plugs and check there condition; should be gray for normal; if white; it is running lean; which means hotter than normal. Black means it is running rich. Also make sure the correct spark plugs are installed. If it is lean; just richen up the high speed carb adjustment. Also a pressure washer can clean up the tractor very well.
