It started as a growl with a slight vibration on the foot pegs. As it got worse, the sound went more toward a clunking sound. You will know this is a problem if you shut the bike down and while in neutral, role the bike backward and forward. You'll hear the clunking sound when you change directions. The sound may come from the front left corner of the motor and resonate to the rear hub. As you continue to ride, the vibration will become worse and much more audible at lower speeds (20 - 40 mph).