I had a similar rattle and it ended up being the left rear rubber swaybar bushing. It was dangling and close to being lost forever and the swaybar would rattle up and down and bang against the bracket that holds the bushing. Could only be heard at low speeds over small bumps etc...reinstalled bushing all is quiet .

 

I'm having a similar rattle in the right rear part of the car. It only happens after I've been driving for some time. I wonder if something is loosening up after it gets heated by the exhaust system.

Any thoughts?