I rebuilt my 99 400ex a while ago cause I put a hole in the case and ran it out of oil. So it basically has a new motor new crank new piston and had the cylinder bored out. Ever since it's been put back together it's ticked on the top end. Sounds like the valves are loose but they're not. Pulled it apart numerous times to check them and they are perfect. They aren't new though thinking they might be bad or something else