What's the banging noise from the rear of my 325?

i've had the car nearly a year and it's great. but last week, half way up the m6, it decided to make a banging noise from the back seat or boot area. i thought it was my son so moved him and his kiddie seat into the front. still bangs. thought it might be junk in the boot. emptied boot, still bangs. a mate thought the side of the back seat was loose, took that out and still bangs. could it be the parcel shelf? all the clips seem to be in place. any ideas would save my ears! the noise sounds like 2 big bits of plastic hitting together and seems to come from drivers side in the back.