Fingers crossed it's an airlock. If so, you can purge it as follows: From stone cold, turn the heater to max, take off the coolant expansion tank cap, then start the engine and let it idle until it heats up, the thermostat opens and the electric fan comes on. Then replace the expansion tank cap. If coolant comes gurgling out before the fan comes on, quickly replace the cap there and then, using gloves and a towel. Be careful not to trap anything in any moving parts.