The door itself rattles, I think the window track is loose or worn out. I read a post once saying that the plastic parts inside the door will wear out and mine is ten years old now.
But the door itself will "rattle". Best way I can describe it is it sounds like you are driving with the door not shut all the way. Only the door is shut completely and you can only hear that when you hit the bumps in the road.
It was suggested to tighten the bolts inside the door connecting it to the hinges, then to check the striker itself. But another thing came to mind. Would the weather stripping cause the door to not shut tightly? Like if its too big?. When I bought the truck I noticed that the seals on each door were jet black and very "fluffy". Like they are brand new.