I had this problem after I installed my SLP loudmouths. It sounded like metal vibrating when i pushed the gas sometimes and sometimes when it was idling in gear. I tried many different things but finally figured it out. The tips were stuck out just far enough that the hanger on the drivers side tip was barely touching some metal on the inside of the rear bumper. I pushed the tips in about a 1/4 inch and the problem was solved. I don't know if this is what you guys are experiencing, but maybe it will help.