If I am traveling along the highway at about 3000rpm, at about 80mph, and I lift off of the throttle, my exhaust begins to make a noise that sounds much like a jake's brake on a disel engine, which sounds like a jackhammer going off. can anybody explain this one? It only happens with throttle lift from high speeds, never at low rpms.

 

Maybe the combination of a slightly loose system and the high speed of the wind does it. Check your exhaust for loose nuts and bolts.