sloshing noise

My aunt has a 2006 Kia spectra with 27k miles. When coming to a stop and accelerating (basically weight transfer) she hears a sloshing noise coming from the dash area. She has taken it to the dealer 3 times and the preformed the following things.

Coolant bleed
Unclogged condensation hose
Fixed and air bubble in the heater core.

All of these things help temporally but the next day the noise is back. The car runs fine and has no other problems. She also says that she does not see water on the ground after running the AC for a long period of time. I am thinking maybe there is a hole in one of these lines and when they bleed it its fine until it builds another air bubble. Anyone have any ideas? Please help me out

you have a coolant leak. Sloshing sounds are air in the heater core.

you have a coolant leak. Sloshing sounds are air in the heater core.
So is there a hole somewhere where the air keeps getting in? Because ones the air bubble is gone its fine for a while and then it comes back

could be a coolant leak anywhere in the system. Could be a blown head gasket, putting air in the system. Could be overheating, causing air to develope when it boils