I am having the exact same problem. I got my 2012 Focus a week ago and it's been in the dealer for a majority of the time. When I accelerate from a dead stop it makes that weird rattle noise. I have had to take it in twice since the dealer claims they can't hear it or isolate the noise. I'm getting very frustrated since this is a brand new car and it's already having problems, and no one at the dealer thinks there is anything wrong with it.