I've never heard it before in my old Fiesta, but for the last few months, the sound of petrol sloshing back and forth behind the back seat as I turn corners is very noticeable. I doubt that my hearing has improved dramatically, and I can't think of any large voids back there where that amount of water might be collecting.
I remember changing the fuel pump in the missus's Ka, and I remember seeing a large circular baffle in the middle of the tank. Is it likely that I have one too, that has fallen off? Maybe some soundproofing has come adrift?