we have a potterton suprima 40 which is about 3 yrs old and just recently been serviced.the problem is during the day and night it makes such a loud bang when the boiler flares up sounds like in the pipes and you can hear the sound of water like its bubbling or boiling but it doesnt do all the time weve turned the boiler and combi temp down checked theres no air in rads and they are fine can anyone please help thanks