loud bang

I have same problem that has been posted on this youtube video: If the boiler has the heating running and you turn on hot water anywhere in the house you can hear this sound. If the heating is off and you turn on hot water, no sound. I also had regular service in November, so 3, 3.5 months...

My grandparents live a bungalow with modern central heating that has a economy 7/dual tariff meter that according to them is making a loudish bang sound when it switches over at night and in the morning. Note that they do not have any storage heaters or any other high night time loads and only...