Weird grinding noise...

I'm hoping someone can help me out here. When I rev my 400ex really high and then release the throttle there is a grinding kind of sound. It happens when it's in neutral or when I'm tapped out in 4th or 5th. Any ideas? The clutch slipping perhaps? If so what do I need to do? Also is anyone going to be out at the Adelanto Gran Prix in So Cal this weekend? I will :)

Oh yeah one more thing - it happens with and without the clutch pulled in. If that makes a difference???

After reading some old posts I think it may be my AC skid plate vibrating. It only seems to happen when I let off the gas at high RPM's though. It's an awful sound and very loud. It first happened after I adjusted the valves and cables, and put on a bunch of aftermarket crap like the skid plate, nerfs, bumper, etc. So I guess tomorrow I'll take off the skid plate and see if it still makes noise. Anyone have any other suggestions? Please :D

or it could be your flux capasitor

[PsychoDave]

Your plastic could be vibrating....check ALL nuts and bolts....my side plastics vibrated and it drove me nuts, so I put blobs of calk on the pannels, let it dry and put it back together. Now there is no more noise!

[Scott]

silicon RTV will silence it.

Also check the Plutonium Space Modulator.
And make sure you have the elbow grease topped off.

[big-guy]

Check the muffler bearings, the blinker fluid, and have the air in the tires changed to a winter grade.

Thank you guys for all your wonderful advice :D It turns out the muffler bearings were vibrating. I replaced them with new ball bearings and some 3-in-1 oil. Topped off the blinker fluid also.

Actually it was my AC propeg nerfbars vibrating. Glad I finally found it. Couldn't have done it without all your help though ;)

yep...we are some B.M.F's...just busten your balls..hope your not mad..good work on the discovery..those ac pro pegs are trickey