The N54 twin-turbo wastegate actuator arms develop excessive play over time. The rattle itself is cosmetic initially but indicates worn wastegate components that will eventually affect boost control.
Symptoms
Metallic rattling noise at idle and low RPM, especially when cold. Sounds like marbles in a tin can.
Fix / Remedy
Replace both turbochargers (wastegate components not individually serviceable on N54). Some shops can rebuild with upgraded wastegate actuators.