BMW Repair: 2004 325ci, failure point, rear shock

Question
Whenever I drive over any type of rough or semi rough road I hear what appears to be something "loose" in the trunk even though everything is secure. I have even checked to make sure the fold down seat is secure - which it is.

Any idea what could be causing this noise? It is very annoying and embarrasing when passengers ask what's banging around in the trunk.

Answer
Likely either the exhaust or a shock mount.

See if the exhaust can be made to move to hit something.

If not, have your rear shock mounts checked.  On earlier 3's, this was a common failure point. I'd guess on yours it's not failed, but could be a loose nut on it.