Yeah, it's probably your muffler mount. But while your down there......... loctite all the screws in your saddle bags cover latches and the two mounting brackets. When I took off the hardware for my new bags almost all the little screws were less than finger tight and a couple way loose.

 

had same prob this summer....siliconed the rubber mounts and would stop..then right back after a big run...bought new rubber mounts and it went away...quick one beer job too!