Mini explosions and rattling pipes


I have a sealed central heating system with a water mains fed wall mounted boiler in the garage with pressure relief valve and plumbed in 4bar expansion vessel. My house was recently extended with an added 6 radiators fitted to the system, and more recently, the diverter valve failed and was replaced by local plumbers. We live in a relatively hard water area. After this work, there is periodically a relatively large bang coming from the boiler area which causes a rattle along the pipes beneath the floor. When the central heating is on this occurs maybe three or four times in an evening, and is very common at the first fire of the boiler in the morning. Local plumbers have been out several times and best chance of demonstrating the banging noise seemed to be by waiting for the central heating to reach temperature and then turn down the thermostat swiftly - bang! along the pipes when thermostat turned down. The plumbers thought that I needed a larger expansion vessel (hence the 4 bar fitted one), but this made no difference at all. I then secured all of the piping around the diverter vale with clamps to ensure no loose pipes - but still we have bangs. Any ideas that I can give to my plumber about what might be the problem here?
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Simon mumbled

Sounds like the water is starting to boil - faulty thermostat on the boiler?
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zaax

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