Gas sloshing sound?

I get this sound when my tank is about 1/4 that sounds like I have a 5 gallon pan under my rear carpet that sloshes when I make turns, brake, accelerate, etc. At first I looked around and found a 3/4 bottle of lemonade under my seat and it seemed to go away for a week or 2. Now it is back and I wonder is it a) the gas tank actually sloshing gas around(never had that in a car before). or is it B) rain water trapped somewhere that is doing it.

Anyone?