Rattling noise from underneath X6

[Solidgold]

Had this annoying rattling/creaking noise coming from under carriage area when going over uneven roads below 30km/hr. Bring it into the dealership and they diagnosed that something in the plastic covering in the tailgate is loose. Fixed that with sound padding stuff. Took it back and within a couple of days the sound is back. Brought it in a second time and they told me that the brake assembly needs cleaning including the anti-roll bars..etc.. Fixed that and still the sound persist. Before I waste more time sending back to dealership..anyone experience similar annoying knocking/rattling sound at low speed? At high speed...or good roads..everything is fine. Sound seems to come from below the seats to the rear section.

Any bros help will be much appreciated.