Sound of water swishing

Probably your hearing coolant flow through the heater core, now this usually caused by low coolant level and you have air in the system. This can also be caused by the engine having a blown headgasket.

I'll assume your car has an AC. A swishing sound is frequently heard inside a car when the condensate tank under the right side dash is full of water. It's caused by the drain being plugged up. If this is it, go under the car, find the drain outlet and run a pipe cleaner in it to free up blockage.

GA correct answer