Overheating car

My car started to overheat while on a drive the other day. I turned the heater on to expel heat from the engine and the temperature gauge dropped down to the halfway mark. I checked the coolant and it was at a proper level and did not seem dirty or clouded. A day or two later I checked the engine, all the tubes seem fine but I noticed when I turn the car off I can hear liquid moving through the engine (not quite gurgling but still loud enough to notice).