well it started when i desided to run my xr600 with out the exhaust for about 10 15 minutes in an open field. I know im stupid but it sounded really cooland alot faster. anyways I burnt acouple valve seals and had to replace them i got it all back together just fine and started up first kick. and now it has a knock well not even a knock its just a tap that fades as it warms up but is still there. on the right side of the head. it runs fine and it starts every time for the past year and a half. an help or ideas to solve this problem would be appricated.