Last year, I had a metallic rattling in the driver's door, only happened with a bit of bass.
Took it in to the Ford dealer for them to look at, they tightened some bolts on the door (ie, they did nothing), but the rattle was still there.
I took the door panel off myself (what a pain in the ass that was) and lo and behold there was a small metal mounting tab rattling against the speaker. It didn't appear to belong to anything so I removed it. Problem solved. Probably an isolated case though with spare parts rattling in the door.
2012 Blue Candy Titanium