i just puchased a used 00 yz426f. It is in great condition. new parts and well maintained. when i looked at it i could not find any leaks or damage and did not notice any problems. It starts right up, runs good and has plenty of power. The previous owner said he had recently had the cam chain replaced. For the few minutes i looked at it everything seemed ok. KBB was listed at $2260. I paid $1700.00 it has some after market goodies and extras with it. I thought this was a good deal so i bought it. When I got it home it started up easily once you know the "routine" and I ran it around for about 30 minutes. then when it was nice and warm i was listening to the motor and noticed a rattle or light knock coming from inside the cylinder. I checked elsewhere for any loose or vibrating parts and found none. The noise is not coming from the top, i dont think its the valves. It sounds like the wrist pin on the piston/ rod is vibrating or loose. Or a bearing? i dont know. Has anyone had this problem before and what do i do?