2004 6.0 only thing done is an exhaust. Every time its put into gear meaning drive and reverse it makes a vibrating causing low power off and on. I personally think it could be the flex plate but you cant hear a thing from underneath you can only hear it and feel the vibration in the cab no where else. Effected intermittently when driving, like when your driving, and before the transmission shifts it causes the sound and vibration in turn effecting performance. Open to suggestions let me know thanks guys hope I can get this figured out