The gurgling sounds that emanate from radiators actually come from air bubbles within the coolant reservoir. These sounds are not normal, and they usually indicate a problem with the engine coolant system - most commonly a broken, cracked or disconnected hose.
Releasing the air by 'bleeding' the radiator may provide a temporary fix, but a broken hose in the coolant system must be repaired as soon as possible. The gurgling noise is a result of the engine overheating and, if it persists, it eventually causes the thermostat to fail. Less commonly, gurgling noises may result from a broken thermostat. In this case, repair is more costly.