Irritating Banging Noise from the Rear

[mobswing]

Hi guys am beset with this irritating sound that comes from the rear cargo section when i hit gravel roads or when i brake or accelerate (which is like always)
It sounds like the upper or lower tail gate is not closed properly or that there is something loose that is hitting against the tail gates. I have properly fastened each and everything that needs to be fastened under the floor mat and ensured that the tail gates are properly shut but the sound persists. Has any of you experienced this or could you please point me in the right direction? Could it be coming from the under carriage of the car but sounding as though it is within the cabin?