Sound at the bottom of the Blaster

[99yamahablaster]

A couple days ago I wen't to ride my quad but then when I was running like at Third gear I heard a sound like if a rock hit the quad at the bottom. So I kept going but the quad was going well. What could of been?

 

[Blaner]

did the sound continue or was it a once off knock? could have been a rock, or perhaps a chunk of piston went out the exhuast...