Drain the oil, remove the side cover and check to see if the flywheel nut is working loose. There was a service bulletin on some bikes for this problem. It is a warranty issue, and the "fix" is to apply locktite and retorque. A better way to go if you do have this problem is to get the special nuts produced by Nichols Manufacturing (50$). They are available from Motowheels, and will solve the problem. Good Luck!