That's what has me baffled. I was riding when the change occured.
Going down the road at 45 MPH and boom...starts buzzing. (Just for clarity...there was no "boom"....that was just for effect).
The bike's been a daily commuter and weekend warrior. Carbs sync'ed. Valves adjusted this summer.
I'm hoping for someone to post up a "Yeah, that happened to me and this was the problem..."
I just did another carb sync....everything was within spec.
If nothing else, I'll start with the valves (God, I hate pulling that valve cover). When I bought the bike last spring, I found one of the valve adjuster screws laying in the head. In that case, I knew something was up because one of the valves was much louder than the others.
Other than the buzz, there was no change in engine sound or performance. Did a few full throttle blitzes just to see if something was getting ready to break. Ran like a champ....a 600cc buzzing champ.