2010 W207 E350 Cabriolet - Fuel pump location

[Johnhward]

I am hearing a low volume ticking/rattle noise when idling and it seems to be coming from the rear end of the vehicle - if I put my ear against the rear body panels then it is loudest
Any idea what the noise could be - could it be the fuel pump maybe and where is the fuel pump located - the noise is loudest around the area of the fuel filler cap