That means there is air in the lines and must be bled out.
If there is air you will hear bubbling as the water flows. If you do not hear anything while the pump is on you are probably ok.
You have air in the system that has to be bled out
My bet would be the hot water heater cycling water through
I think if the automatic fill draws water from your home water service it would not be unusual to hear a "running water" noise when water was being added to the pool.
I would check the toilets, that is about the only place water could be getting into the drain without seeing it.
Sting. Set them Free (if you love somebody)
Generally the running water sound under the dash is air trapped in the coolant system. This air then circulates through the heater core. Just check the coolant level in the radiator and make sure the overflow reservoir is full. Run a few heating and cooling cycles of the engine and recheck. This should help take care of the sound.
Your Heater Core is most likely leaking. Your heater core is under the dash inside the car which is why you can hear it, and also why you need to continually fill up the water.
You can hear it running. Unplug the fridge, then wait approx 1 minute and plug it back in; you should hear it kick on.
they can hear better out of the water
Convection
chances are you don't have it full enough and there is a bleed valve by water pump