DIY Doctor UK DIY Forums

My central heating system, powered by a Baxi Platinum combi boiler, is very quiet when heating from cold in the morning and also when cooling at night. However, when the room temperature reaches/exceeds the set point, the thermostat continues to fire up the boiler with monotonous regularity, sometimes 6 times an hour. When this happens rapid tapping noises can be heard, from several points in the system, during the periods immediately after the boiler firing and shutting off. This is not only irritating but also a waste of expensive gas. My service engineers put the noises down to expansion/contraction, but, in my mind, there isn’t a sufficient time interval for significant cooling. I’ve bled the radiators to no avail and the pipes are under Weyroc flooring, which makes inspection a problem. There are no abnormal noises from the boiler and pressures are static.
Anyone got any suggestions.