my car did that same thing at about 2500 rpms, does it sound like a couple of metal pieces rubbing together real fast, kind of like a TWIING noise.... on my car the bolts that hold the heat shields onto the cat conv rusted out, and the shield was loose, when the engine hit the perfect rpm it would cause the loose sheild to vibrate and make this god awful noise.... cut the heat shield off... noise stopped just like that.... check all your heat sheilds, make sure they are tight, if they are rusty and or loose, find a way to tighten them down, or remove them all together.

hope that helps, hey if i turn out to be right drop me an e-mail

boxxer@chartermi.net