Need help Diagnosing Rattle

Need help Diagnosing Rattle***update 3/8***

Recently I had the Ford Racing X pipe installed (splines into where old H was) and since then I have a rattle at 1000-1800 rpms...initially I though resonnance but it sounds like ass...metal on metal soft clanging that paces the rpms of the engine at idle. Its not real loud but I can hear it inside the car over the drone of the exhuast...after 1800 rpm its gone. It is also more prominant when I am going up hill on an inclined driveway or making uturns. Yesterday I had the car stopped on the incline of my drive way and I could hear it...so I engaged the parking break and checked it out...it sounds like something is loose like an exhuast hanger or clamp but I checked everthing and its pretty tight everywhere...I have no clue where its comming from but it is definately at the back of the car, any help before I let a dealer look at it would be great, thanks

So after taking it to the dealer I drove around with my advisor and sure enough we could both hear the clatter...I mentioned TSB 06-4-1 and he said it sounded more toward the back end of the car which does not adhere to the symtoms related to the TSB. What it ended up being and I am not sure if others have had this same issue with Stingers (FRPP or Borla) but the rear most hanger bar on the muffler points straight backward toward the rear of the car...in my case it was butting up against a piece of the frame I think there is a bolt there also. At idle when the car is cold the exhaust contracts and it does not hit the frame/bolt but after driving around the exhaust heats up and expands backward toward the rear end of the car causing this hanger bar from the muffler to bang against this frame/bolt at low vibrating rpms......10 seconds with a torch and that took care of it.