water sloshing in passenger dash?

You may have a coolant leak. I would think that would have been checked when the coolant was replaced but maybe not. The low coolant level causes air in the system and the water sound behind the dash. If you find the coolant low again there must be a leak to be repaired.

Mine started like this and before it was over with the dealership was putting a new head on the motor (Their dime yippee) Now the sound is back less than 6 months later.......