Mine does that too... my problem was that one of the muffler
hangar arms sits real close to this metal hook thingy
in the rear. When the exhaust vibrates at around 3500
it makes a gross metallic rattle.

I actaully put a piece of my foam mouse pad in bettween two
see if that was where the noise was coming from.
It stopped the noise... now I just gotta come up with a
permenant solution.
:dunno: