The sound may be air mixed with coolant running through your heater coil. I would suggest checking your radiator fluid and see if it's low.
I installed a Stillen all-aluminum radiator that developed a pinhole leak after about 3½ years. I first began hearing the sound of rushing water inside the truck interior prior to finding the radiator leak. I ended up replacing my leaking radiator with another all-aluminum radiator and once all the air was removed from the engine block and heater coil, the sound of rushing water was eliminated and now runs silent.