Just got back from a night drive and since it's late I just drove really slow.

I had my music playing really quiet and then when I stopped at a light, all I can hear is this water swooshy noise....I thought it was my car cleaning stuff in my boot but it wasn't coming from there.

I also though it was the coke bottle that's unfinished in the back, but it isn't....I then put my ear towards my gear stick and can hear it from there.

iirc, isn't the gas tank underneath the car?