My car has a rattle in the trim that runs above the drivers door. It's right next to my ear and drives me nuts a few days a week while commuting to work. I usually smack it once or twice and it stops. It's been doing it for two years now. My last Mustang had a rattle under the dash that never went away.
I recommend you make an appointment with your dealer for diagnosis as soon as possible. Then, PM me with your VIN, dealer, mileage, full name, and best daytime number so I can get this escalated to the customer service manager for your area.