I have a mix of about 50/50 107 octane turbo blue and regular 91 octane. No difference in the noise. Still there.

Please, everyone THINK! WHAT COULD POSSIBLY MAKE A NOISE IN THE TOP END AT HIGH RPM UNDER WIDE OPEN THROTTLE **ONLY**!!!! I have a glamis trip planned in a week and a half, and I must go. Please, people. Any ideas are appriciated.
The bike is also for sale and the ad comes out in one week, tomorrow. (thurs)
-Matt

 

in not sure if its been mentioned, but have you checked your valve clearences?

Honda 400EX Riders Site

 

Matt,

Sorry to hear you still haven't figured it out. Valve clearance MIGHT be it, but if so, it would rattle ALL the time, not just under WOT.

 

Hmmm...Did you check your cam-sproket bolts for proper tightness? How is your spark plug to piston clearance at TDC? How loud is it?