Whaap, you just described the problem exactly. Mine is coming most noticeably from the left side. But, like a tuning fork, it is hard to pin point the sound. It isn't that big of a deal, but I just don't want something to break on me. Usually sounds that shouldn't be there indicate some sort of a problem.

 

What I'vebeen toldis the next time you detail your bike, clean the belt and pully of belt, dirt dust and maybe leave a little soap lubricant on the edges of your belt (not the face or teeth of the belt).

I'm going to wait till our weather gets hot-hot and if it starts that resonance again I will try the above and see if that takes care of it.