If you took of your rear wheel,it's the hard rubber bump stop mounted at the top of the wheelwell.Bump stop is the only word I could describe what it would be.One of them is gone and the other is falling apart.

This could explain all the banging I hear in the rear.

 

The part # is 10164173. But you only need this in extreme bottoming out conditions. Its probably not the cause of your rattling. Good Luck!

 

Thanks Dave!!!!!