Ok so 46 people looked at this and had no solution. Maybe I diagnosed this wrong. I looked all over this forum to find noise repairs and most had to do with exhaust mounting. I took the pipe off and found the gasket between the pipe and cylinder had worn away. I also found one broken mount and that the pipe has been vibrating against the top of the shock for a while. I replaced the gasket and o-rings, let the pipe hang by its springs, tight against the cylinder, then shimmed and fitted each mount. A little grinding on the shock casting, then off for some riding. Now the bike is quieter than it was on the day it was new. Love this bike!!!! After riding this bike for a year with a flywheel weight I removed it two weeks ago and love the big hit, my arms are now strong enough to handle it. KX500 Rules!