I spent ten minutes crawling around my bike when i first noticed the "buzzing" sound, almost like speeding along with a clothes peg and hockey card on my spokes.

Anyway Try the mud shield that's inside your front wheel well. Towards the front where the plastic is Close to the vertical bumper bar (around the wheel well side from the bush plate, radiator guard thingy)

Mine usually comes on when the fan is on!

 

If the Prairie 650 is like the Prairie 400, then like Steelhead650 mentioned it may be the black plastic shields inside the fenders around the shocks. My 400 Prairie was doing that. I noticed it most when letting off of the throttle, so I put the transmission in neutral and revved it up and let off the throttle while pressing on the different parts until I found exactly where the problem was. On mine, it was the mud shields vibrating against the frame, so I drilled holes in the plastic and used tie wraps to tie them to the frame more tightly---end of problem.

 

Thanks guys, Im heading out now to ride and see what I can find.