ok your problem is the caliper that hold the brake pads. Normally they should fit really thight and not move. I used to have the same problem. It is nothing critical, don't worry, nothing will fall out or anything. The noise is very annoying though i must admit.
If you wanna buy new calipers, it can turn out to be quite expensive. Over here in england they are 150 pounds a piece, so about 600 bux for both sides.
What i suggest you do is take out the brake pads. You will see some thin metal brackes on theonside that go into a hole in the caliper. Just make those a bit wider by hand and put some heat resistant grease around the corners.
That is what i did and everything is fine now there is no rattling at all anymore.
Another thing you can do, but i do not reccomend this one is just to put a welding spot on both sides of the caliper where the pads are suppose to lean on, it will create tha half a millimeter you need to stop the rattle.
Hope this helps