More than likely, I would agree with the second poster that the issue is a loose chain caused by a failed chain tensioner. In general, the chains on these motors are very stout and will run for well over a 100k miles. The tensioners, though, do fail and can be replaced.

My recommendation would be to change out the tensioner.

If you elect to change out the chain, it is not a very big job; but, does require a few special tools not usually owned by the DIYer. (Dremel tool to cut a link and a chain link press).