Thanks for the helpful response. I've discovered a bit more information that might be helpful in identifying fixing the problem. The chattering stops or subsides when the bike is tilted in either direction, and particularly when it is on its kickstand. When it is in neutral but in an upright position, it chatters just like it does in first and second. Only when the bike is on the stand does it require very high RPMs for it to chatter in neutral. I had a friend sit on the bike today while I listened for the source of the noise, and it appears to be coming directly from the gearbox. I cannot be 100% sure of its location, but this makes me think there was a problem with the clutch installation. Do you think this will be complicated to fix? Can I do it myself, or should I go to a professional?