I have a better description of the sound, a metal-to-metal vibrating , but it is not a deep sound, think of nails on a chalkboard, but metallic. When the engine has a load (pinning the throttle) it quits making the sound, like as soon as it "hits" the pipe, and smoothes out it quits also. It's sort of a harmonic-chassis-vibration. I'm clueless. I'm going to check ALL of the chassis mounting points for everthing AGAIN and see if there's anything I've missed. Thanks for all of your insight as well.