Both my front doors have this problem...I was able to alleviate it somewhat by removing the door panel and slightly loosening some of the bottom bolts that hold on the inner door panel. While doing this, ball up your fist and hit the flat surface of the metal door skin at the extreme bottom inside edge of the door...this duplicated the rattling sound for me....then I backed out a couple of those bolts and the rattling went away. I occasionally get a rattle while driving, but it's not constant as it was before I did this.
I think there's another layer of steel inside the door that's there to thread the second door panel bolts into. When you tighten the bolts all the way down, it seems to bring that metal closer to the inside door skin and any vibration causes them to rattle against one another. That's the only explanation I can come up with for the rattling to go away when I loosen those bolts. My other piece of advise: If you haven't messed with it yet, take it to the dealer first. I have a feeling they won't warranty it if you've already tried to fix it yourself.