What kind of air filter are you running? I have a differernt friend (than the one that chased the opposite end of the car) that was trying to hop up a 6.2 diesel, he made a flat plate to replace his factory air filter lid. Turns out there is a harmonic (on the 6.2 anyway) right where it cruises that made the new lid reverberate loud enough I could hear it from behind his truck on the freeway. Yours doesn't sound like it's this loud but a different type of air filter might help (if it's the problem).
