Welcome. This is usually caused by a plugged it drained for the air conditioning evaporator. On the outside of the car, under the hood, lower down on the firewall is a rubber elbow that connects to a plastic tube to that is the drain. Often this drain gets pinched or plugged up with debris. This allows water two buildup inside the air conditioning system that is likely making your noise.
Another possible cause is a low coolant condition. If there is air trapped in the heater core, you will get a sloshing noise depending on what the engine RPM is doing. First I would check your antifreeze level and if it is topped the proper, that I would find this robber elbow. You can stick a piece of wire inside two free up whatever is causing it to be restricted.