Rattling in reverse
 

04 AT 43k miles.
Just replaced coils, plugs, added catless midpipe.

When the car is in reverse or sometimes in park it rattles fairly loudly. I thought it was the cat but obviously it isn't. Help.

Does this come from the engine and go with the RPM's? Or does this come from the wheels?

If engine, I bet it's the Motor Mounts. I believe this was an insanely widespread problem with 04's, and some 05's.

At least take it to a dealer and see. The engine has an 8 year/100,000 mile warranty on it. Tell them to check it out.