Rattling in rear under hard acceleration

[Intel55]

Do you have the load floor or the tow hitch?
License plate or holder.
Heat shields. There are four botls that hold each heat shield and are easy to check.
Loose PDC cable?

There are only a few things under the rear that you will be able to look up and check. The other could be loose nuts on the exhaust pipe and muffler holders. There are 5 nuts on each side, two under the lower body mount and three near the bumper.