I am not familiar with your specific vehicle, but I have encountered similar issues when all there was air in the cooling system.
The heater and defroster works by using a heater core which is like a little radiator that sits under the dash, then a fan blows air over it to heat the car. The biggest problem is that this heater core can sit pretty high compared to the rest of the cooling system and it is easy to trap air in it. What you are hearing is most likely the sound of water moving through a larger air pocket that is stuck in there.
The first thing to check is how much coolant you have right now. But only open it when the engine is cold! In your description you said that you did this, but did you just look at the overflow bottle or take the radiator cap off?
If that looks OK then you need to look up the purge procedure for your car. One of mine actually wants you to jack the front up and push a couple of buttons on the HVAC unit when starting it to put it into "Purge Mode. Most of the others I have are just, turn this little screw and let the air out sort of things.
Once the air is out you should not hear anything as the water moves through the pipes.
As an aside, turning the heater off does not necessarily prevent water from circulating through the heater coil, many don't regulate it or use a fixed thermostat independent of the heater settings.