If you are putting it above 4000 rpm and letting it coast there, that in itself is not too much of a good idea. lol but thats your choice


I would have a tendency to think that your friend could be right. High RPMs can cause certain vibrations, and if you have a heat shield somewhere that is loose, due to slamming the underside coming off a road bump or something...


Does the pitch of the sound change with the engine's sound pitch? if so, then that would make it related directly to the engine. If not, by guess would be a loose part... like I said, probably a heat shield.