Too loose a chain can slap the chain guard. Secondly, check the front sprocket as the sprocket nut has been know to come loose. Thirdly, check the chain guard mounts.
As Friar says it could be in the top end, but check the simple things first. You might want a friend to listen as you drive by, sometimes a bystander can pinpoint the sound's origin easier than the rider.
Good luck!
