Sometimes when I start the car it rattles in the back for a brief moment. What could this be?

 

That sounds alot like what my cat is doing right now. I thought it was coming out of the rear end until I really put my head down by the bottum of the car. It does it when I start the car, when I turn it off and if I lug it off a start. The other thing it might be if it really is coming out the back of the car is your gas tank shield.