Loud banging/rattling SL350 Fuel Tank?

I have a 2004 SL350. There is a loud banging/knocking sound from behind the drivers side seat which I first thought was something in the boot or roof mechanism but now I think its to do with the fuel tank as the regularity and intensity varies depending on how full the tank is.

Does anyone have any experience of this and if so how and how much was it to fix?

Thanks in advance
Luis