Rattling

[Apac102]

The back wheels in my car are rattling everytime i go over a lil bump in the road. At first i thought it was the exhaust hitting something but it isn't. Then I just went back and forth over a bump and i located it to the back wheels. It goes Klanky klank klank (if you can make sense of that). BUt Im just curiuos if anyones has any idea what this is.