Sounds like the same problem on mine??
I was told by Zentrum that it is the "Lower Control Arm Bushes" that are the cause, they crack or corrode and will rattle like mad, especially if you drive over cobbles. It should not cause any handling issues though.
I was also told that you can't physically see the problem on the bushes and it only manifests when assembled and the car is moving.
http://www.hartech.org/parts.html do a £90 replacement part which I will try someday otherwise its a £200 part if going original.
Was told to get both rear sides done as they often go together.