The Red October

It happens when the car is in gear and rolling at slow speeds... Like in a parking lot. The rattling happens as soon as I give the car a small ammount of throttle to slightly accelerate. It will only rattle for about 1/2 second, and it is easily heard with the windows down. Sounds very metalic. It does it at RPM's anywhere from 1500 to 2000.

I also have a similar sound on cold starts where the car has been sitting all day. It seems louder in the summer months, and the rattle only last for about the first 1/2 second that the engine is running. Very metallic sounding, like a rock in a can.

Any ideas what this is? Exhaust rattle? Where should I look?