Flatlands Skid Plate vibes...
I put a Flatlands Racing skid plate on mine right after getting the bike 22K miles ago. It had harmonic vibes that were somewhat irritating to me. Next oil change while I had the plate off I reinstalled with 1-1/2" rubber washers placed between frame and the plate at the four bolt hole mount sites. No more vibes. Rubber washers were about 35¢ each at the local ACE hardware store.