It could just be the gas sloshing around iin the tank, which is, obviously, normal. If the radio is off and the outside environment is quiet you can hear it faintly sometimes. It's quite noticible if the rear seats are folded down.
On the other hand, check the floors and trunk thoroughly for any wet areas. If everything is dry, however, it is probably just the gas tank.
On the other hand, check the floors and trunk thoroughly for any wet areas. If everything is dry, however, it is probably just the gas tank.