That is where the heater core on most automobiles is located - just inside the passenger side dash. When you turn on the car's heater, water from the radiator is passed through the core under the dash, heating the air that comes out the vents. It's perfectly normal on some less-expensive vehicles to be able to hear that "gurgling" due to lack of noise reduction material in the cabin. However, it can sometimes be a sign of overheating due to running low on radiator fluid or a faulty thermostat. Check the fluid levels in the vehicle and have the thermostat system checked.