Sharp cracking noise

[fordgt40]

I have a c63 estate and when going over certain types of bump it emits a 'sharp crack' almost like a pistol shot from somewhere in the rear, sounds like it's coming from the roof or side panels, I've tried everything, tailgate open, rear seat belts you name it, we've tried to isolate it but without success, it's booked in the the dealers to try to sort it out but if somebody could point me in the right direction as to what it could be it would be most helpful.