Water sound in dash

I have come across a couple things that could cause a water sloshing noise: First check the coolant level of the cooling system. If it is low, suspect a leaking heater core, but since you did not mention any coolant smell or fogged up windshield, I have to eliminate that theory. Try turning on the defroster or A/C for a few minutes (both activate the A/C compressor) and observe if any water drips from the evaporator case drain tube. Like a cold glass of water, the evaporator condenses water in the air, which is drained out a tube by the firewall. If none comes out, it may be clogged which could be what you are hearing. Another concern could be a water leak from the windshield area collecting somewhere in the dash. Of course, a “Big Gulp” left in the cup holder makes sloshing noises. I hope this helps.

heater core is leaking and u cant see any leaks cause box is sealed