I had the same creaking sound when making a right turn onto an elevated driveway. Here is the fix,
My dealer in Serramonte identified it as the rear seats having 12 screws/bolts being mounted too short at factory. Due to the stiffness of the car, these bolts got jarred loose, therefore creating the styrofoam rubbing noise. He changed out the 12 screws/bolts to longer ones. Never had the problem occur again ever since. My FX has finally been rattle free.
I had the armrest rattle initially. They fixed that with padding, that did the trick.