I too had this problem, I just never bothered to try and fix it as the car has cost me enough already.

My cars make this sloshy sound when I hit the gas or stop quickly. At first I though it might have been my gasket, but it is definitely right behind my radio area. it is more of an annoyance than anything. At least I hope. I'm going to wait it out. I hate my car.

when you drive there is a water sound coming from the dashboard and when the air is on it sounds worse. I took it to the dealer today and they said that if they had to go in and drain the box behind the dash it would cost 75.00 bucks. If the were to remove the dash and completely take it out to find the problem it would cost 300.00 to 700.00 dollars havent checked the extended warranty but hoping that it will cover it but if not it will be traded in next year.