I only hear this rattle when I have the windows open... it doesn't resonate in the car or through the wheel/pedals.

Going over bumps at low speed you can hear it pretty clearly, it sounds like it's coming from the bottom rear of the car. Any ideas what it might be?

 

My friend had this problem. Turns out one of the shock bolts was loose. Check those.

 

I just had acar with a similar noise turns out the rear brake pads where loose. Take a look at them too.