Sounds like you've got an airlock in the cooling system. Run the engine with the expansion tank cap removed and massage the coolant hoses all around the engine bay particularly the two large ones nearest the radiator which should then force the air bubbles up and out of the tank.
It may take a few times to get the air out completely whilst monitoring the water level and topping up as required. Another method is to level the cap off the tank over night and some people have reported that the air has eventually found it's own way up and out on its on accord.
