Just recently acquired a 2010 Subaru Impreza this summer and it's been great, no strange noises or problems.  However, I went to start it last night when leaving work and I heard this horrible rattling sound from under neath the car.  It was 8* when I left work so that probably has something to do with it.  I let the car warm up, and it went away.  It sort of sounds like this video, except for it also did it in park:
http://www.youtube.com/watch?v=qKi4WrnbdaI

The noise reverberates throughout the car, but it almost sounds likes its coming from directly below the drivers seat.  Any ideas?

auto or stick

It's automatic, and I forgot to mention, it only does it when stopped.  When I accelerate the noise completely goes away.

Possibly the exhaust rattling...

Sounds like an exhaust heat shield, same thing happened to my wifes Impreza.

Tag since my wife's '09 Impreza started doing the same thing yesterday.

My 2010 Impreza does that and has for over a year now when the engine is cold and it's worse in cold weather. It is the exhaust and I just haven't got around to locating the exact spot and trying to fix it yet. I've read that Subarus are known to have that issue.

Heat shield.  Just cut it off.

heat shield rattling.
https://www.youtube.com/watch?v=-tzQclY43IA
1. Go to home depot and get some big stainless steel hose clamps.
2. jack up car and use jack stands
3. wrap hose clamps around heat shields and tighten very tight.
4. ???????
5. drink beer and profit