It will be the doors as michael says, there is 3 bungs in the bottom of each that get blocked up and they start retaining water, just remove them completely and be prepared for a good pint of water to come out
I've never once heard the petrol in my tank but my doors were full of water once and I could hear that clear as day
depends on the age of the car (only upto 07 models i think) but the bungs at the base of the boors can get bunged up and stop water flowing out. just take em out n chuck em, you dont need them, the doors may well be full of water. I've never heard the petrol slosh about in the tank, ever.