I had a similar problem with the VW Passat I had at the time - the drain hole from the battery compartment got blocked and when it rained it overflowed into the interior of the car - I was wondering for ages why there was this sloshing noise each time I went round a corner - the back of the car had about 1cm of water in it!

Had to get the dealer to remove all the seats and carpets and dry thing out.