Preliminary data based on the notification from the Icelandic authorities:
The aircraft was being ferry flown with payload ballast in the aft most position (position 10) in the main cargo hold. During descent towards LFPG there was a huge bang that shook the entire aircraft, producing a strong yaw and destabilizing the flight path. Investigation by the flight crew indicated that the payload ballast had moved from the aft most position in the main cargo hold and towards the G9 wall in the forward most position of the main cargo hold.
During both flap and landing gear extension the loose ballast moved again and slammed against the G9 wall.