[richtazz]

The only other thing I can think of is is you have a cracked upper intake plenum, and the water is inside the upper intake, but I wouldn't think that would make a sloshing sound in the car. Do you lose coolant? If you're slowly losing coolant, I would say to have your upper plenum checked for cracks where the throttle body mounts to it. If you're not losing coolant, you have me stumped.