i have the same sound. it's the subframe bushing. there is a TSB on this issue. here is the exact text from VADIS:

'A knocking noise that may occur when driving at low speeds on uneven road or at sharp turns, can be caused by one of the the rear sub frame bushings moving in the sub-frame. The knocking can sometimes also be felt in the floorboard under the drivers feet.
Install an O-ring between the sub-frame and the bodywork on the rear bushings on both left and right side as a service solution.'

new o-ring's are required. it's a relativily easy fix. i haven't had time to get to replace them it. although the sound has diminished in the past month.

hope this helps.