Maybe not your fuel tank...
I used to have the same problem in another vehicle (not a Benz). The noise was most evident when stepping on the brakes and it sounded like water was slushing around and rushing forward. Apparently, it was water in the channels in the door frame/floor where the wires normally go. Water was seeping in whenever it was raining hard or whenever the car was washed thoroughly. Sealing it properly eventually solved the problem.
You might want to check that out.