Sit down in the drivers seat with your legs out the door (like you're getting in/out of the car), remove the door panel.
now swing the door in an open/close motion, making sure when you change direction you do it quick to simulate the door closing.
if something's rattling, you'll find it!
Now, when you THINK you found the issue, isolate it with your hand, then swing the door again in the same fashion, if there's no more rattle (because your hand is isolating it) you got it,
HOWEVER, if it still rattles, keep looking,
grab, swing, repeat till you get it.
Used this technique many times.
*ONE word of warning!
DO NOT swing the door all the way open (till it hits the stops, you can damage your hinge bushings) and DO NOT close it on your legs! It hurts...