Bizarre plasticky knocking noise - help!

[jonnykerm]

I've got a strange noise going on in my E91 which I've just bought. It's coming from the front passenger side, specifically in the footwell closest to the front of the seat and to the right hand side. It sounds just like a cd case sliding in a glove box left and right when I turn corners but I have NO IDEA what it is. Does anyone have any ideas? I can't see or feel anything. It's doing my bulb in.

Thanks in advance.