I had the same thing on my CBR and was told it was the clip holding the front sprocket. Or something like that i was in need of a new set though. Hope that helps.
Could be the nut securing the sprocket. Yamaha Fazers suffered that problem in the past. In those cases (pre-'04) the nut was too weak or the thread to fine for the torque.
Couldn’t say for sure, but it depends on the state of the thread. If that is worn out, it might need to be re-cut (? ze bl**dy word is missing in mi wokabulary ).
Normally the nut is secured with a washer and tightened using a torque wrench. Don’t know what Yamaha thinks about loctite, I’d probably refer it to a competent garage if in doubt.