This can be caused by two main things. The first is air in the cooling system. In the second half of this article, I talk about how to handle this issue. In addition to air in your cooling system, you could have a blockage at the drain tube for the AC system. Under the evaporator is a drain that is supposed to allow moisture from evaporator condensation to exit the HVAC system. It's the reason you see your car dripping water during the summer when you run your AC. This is due to the humidity collecting on the evaporator in your HVAC system. It's the same thing that happens to a glass of ice water when you let it sit out. The AC drain is there to help remove this moisture. If it becomes clogged due to debris, it can allow the water to collect in your dash. This can manifest as a swishing or sloshing noise when you make a turn, and in some cases you might even get water leaking into the passenger compartment.
This is actually really easy to fix. All you need is some compressed air. Find the AC drain tube and blow compressed air into it. Watch out; once you clear the blockage you might get a bath. One way to help prevent this is to remove any debris you find under your windshield wipers at the base of your windshield. This is the fresh-air opening to your HVAC system. Any debris that collects here can work its way into the HVAC system and cause problems like this. It’s less likely with vehicles equipped with a cabin air filter, as those filters are designed to trap debris before it gets into the HVAC and causes a problem.
Here's a video on the process of unclogging an AC drain.