Oh, hey, ok. That sounds exactly like my issue and no, it has not always rattled like this. I've owned the car for 3 months now, and it only started doing this a few weeks ago.
Seems to me like there is a loose piece somewhere in the exhaust system.
I've been thinking of taking to dealer, but im afraid they will say that I probably "hit" something and so warranty wont cover. But i'll probably take it anyways, i hate the sound. This v6 ecoboost sounds beautiful on idle, nice deep tone, and the rattle makes it sound like a cheap old car.