Like Big B. said, probably an accessory bolt or a bolt on something you have worked on recently. Or... your engine is about to grenade itself. (Just joking)
I had a slow speed drop about a year ago and unknowingly sheared a bolt that held on the crash bars. I didn't notice any problem for a while but it manifested itself as a buzz at higher RPMs. I was able to diagnose mine by putting my hand on various parts of the bike while riding and finally I found the part with the extra vibration. Does your buzz happen all the time all the time or just when you are revving it pretty good. If it does it while the bike is stopped, you can more easily find the problem.
Current: '12 Dl650, '06 ST1300, '89 Transalp, '84 XL350R, '07 TRX350TM,