There is air in system, undo big nut in middle of pump left open for 1/2 hrs , open air vent on radiator one by one ,open drain out point let water comes out slow flow when pump be come quite than you notice no air ,close all operation it depends who the pipes are run some time it takes 4 to5 hrs to eliminate air from system while you do all that switch of boiler from thermostat which is on boiler
ltd
March 2013