Knocking by passenger side front

[timjbart]

Ever since I've had my 320i I've had a slight knocking by the front passenger. It sounds as if a nut has come loose and is rolling around somewhere and knocking against metal. It seems most apparent between 40-60

Took off the passenger door trim but there's nothing inside it, looked behind the left speaker but I can find anything. Looked in the grill at the bottom of the windscreen but can't see anything.

Does anybody have any ideas? Thanks