Sounds like a plugged evaporator drain. If you can get under the truck, you will see a tube from the firewall or HVAC box. Try to dislodge the obstruction by inserting the erase end of a pencil or similar blunt object. Some drains have a 90° rubber elbow on them so it may be necessary to get the truck on a lift.
The only other sloshing noise that I've run across was with an air pocket in the cooling system and you could hear the coolant through the heater core.
What we've got here is... failure to communicate. Some men you just can't reach. So you get what we had here last week, which is the way he wants it... well, he gets it.