Rattle in front drivers door

My 04 Focus 1.6 Zetec 5 door has developed an annoying knocking / rattle in the drivers side front door. You can hear it when turning a
corner and when closing the door. In fact you can hear it when standing outside the car and swinging the door backwards and forwards whilst stopping it from actually closing.
It sounds like there is something substantial loose in there, but that might just be my imagination. At first I thought it might be something rolling about in the door pocket like a CD, but I emptied it and the rattle was still there.
I had the regulator replaced last year, could that be causing the rattle?
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