Recently, my 2006 530i has had an odd sound coming from under the rear seats. Whenever I start or stop, or hit a speed bump, I hear the sound of water sloshing around in a large bottle. This occurs whether the tank is full or empty. I have had the car nearly four years and have not heard of this before. Is the gas tank under the rear seat? Is there a bladder in the tank that shrinks as the gas is used, thus keeping the gas from making this noise? If there is a bladder, could it be damaged?

I checked everywhere in the passenger compartment and have no loose water bottle in the passenger compartment nor in the trunk.

Could it be possible there's a tiny hole in the tank and the sloshing might be coming from the outside? I dunno, it hasn't happened to me, but make a note and see if there's a trail of gas when you pull into the garage.

this happened to me in my E90... the sound stuck around for a couple weeks then went away... it has to do with the pressure in the tank... the fuel tank is pressurized, and when there isnt the right amount of pressure in the tank, you get that noise... after a couple more re-fuels it should go away, just make sure you always tighten yor fuel cap until you hear it make a click sound.. i hope this helps

They make special foam blocks to shove in the tank to prevent sloshing. Its mainly for track driving for people who don't want to weld in a tray to prevent fuel starvation.

Thanks. I'll pay attention to that detail.

Ihave this same exact problem, and iv found the answer, my rear passenger door was full of water! somehow rain has been getting in and being stored in the inside of the door! if you llok right at the corner of the door on the hing side you will see a small hole, jack the car up and the water will trickle out!