Air Pocket....one thing I was taught was to leave the water neck and thermostat out....fill the system through the water neck until you can see it at the top...put the thermostat back in and attach the water neck....continue to fill the remainder from the radiator opening and this will eliminate the possiblity of an air pocket in the block or heads....now I have an electric pump and I drilled two 1/8" holes in the thermostat...I filled it and run the pump for 5 minutes until I saw no more air bubbles at the radiator opening...