motocrossmann

I'm getting a high pitch flutter sound that, so far, only seems to come when the tranny is in 3rd gear. If I rev the motor to 5000 or so in 3rd gear, then just let off the gas I get a fluttering whistle sound for a few seconds. I can't get it to happen in any other gear, and if I depress the clutch in the middle of the sound, it immediately stops.

Ideas? I can't envision what might cause this. It sounds like it is coming from the tranny area, but it is hard to be sure. The car has been tracked a few times, with a few missed shifts here and there. The shifter action feels perfect.