I'd bet you have a bent heat shield that is rubbing on the exhaust somewhere. Have some one be in the car and have their foot on the brake while in gear, set the e brake, and chock the wheels. Then lay along the side (not under) of the X and see if you can hear the general area it is coming from. Then with the engine off, go back under and scout out the exhaust and it's shielding for rattles. Don't be afraid to bang you fist along the pipe and shields. You just might find it.