if it is the same noise i have heard on mine, it seems to be coming from above the engine, and it sounds like something is rattling, like maybe one of the plastic parts or something, but the more i ride mine, the less often i hear it. agree about the certain rpms, still havent figured out what it is and i was quite alarmed the first couple of times i heard it, because it almost sounded like a piston slapping or something. i ride without my helmet and have leaned down there to try and hear it a few times and have decided it is plastic rattling only when the engine vibrates at a certain rpm usually on hard acceleration in higher gear......anyone figures it out please let us know