Dashboard Rattle! That I can hear the majority of the time I am driving?

Ok so I have an annoying rattle that I can hear the majority of the time I am driving. It has been there since I bought the car and has gotten slightly worse. I do not believe it can be heard from the outside. The sound comes from the passenger side of the dashboard (area). It isn't constant though. I mostly hear it when I am accelerating, at any speed, but it tends to get louder the faster I am going. It isn't TOO loud, most people wont notice it unless the radio is off. I also sometimes hear it when I first start the car when it is cold. The sound sounds kind of like wind noise but it isn't, it's a rattle. This is the best description that I can give of it. Please someone help! It is a 2007 ford focus auto. If it helps any, it doesn't make a difference if I have the air on or not.