When my boiler turns off with the stat having reached its desired temp, after a minute or so there is a very audible sound of water rushing into the pipes at the top of the boiler. Its as if there is air in the pipes which then at some point is filled with water. It doesn't do this everytime though the boiler switches off. Is this normal and if not, what can I check? Should the main feed and return pipes to the boiler have air vents on them?