The guy in the trunk is still alive!!!
Dynamat
no oil
The rattling you are asking about turned out on my 320 to be the catalytic converter breaking up internally.
Is your oil level?
the muffler rattling? the muffler rattling? the muffler rattling?
Assuming that you have checked the fuses, look for a loose connection in the wiring harness. I believe that there is a connector in the trunk area that may have come loose.
Possibly a loose heat shield or other component.
C5 has a very large trunk area.
Check the emergency trunk release handle. It is in the trunk area, at the rear of the vehicle. If it is not secured in the clip, it can get hung up on something in the trunk area and keep the trunk/hatch from closing. -Tim
Two main causes: the inner parts have failed, or the outer heat shield has failed.
bad wiring