Sloshing sound dashboard

Hello, The answer is that one of them is correct, and you will be able to tell by the smell of the air coming from the vents. If it smells like coolant, it is the heater core, if it smells like mildew or nothing, it is a clogged drain plug. This is so common that you should be able to find a picture online of the A/C drain plug, and stick a small screwdriver into it.