For the past month or so I've been getting quite a pronounced rattle coming from the rear of the car..most definitely caused by something in the interior

it sounds just like the parcel shelf moving or something in the armrest rattling about, however it isn't being caused by either of these sources...
I can hear it particularly when driving over rough/potholed roads and when turning off the engine, and I'm 95% sure its something in the back but can't think what!
Has anyone else had this?
