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Thanks for the responses. The system has been vented and this has not made any difference. Its definitely not a gurgling noise, its a banging clanking sound that echoes all round the house. The boiler is a Glow Worm Hideaway 100 open flue. The noise is coming from inside the boiler itself not the pump. The noise starts when the water temperature approaches the upper thermostat setting and then when the boiler cuts off the noise stops immediately until the next cycle. The higher the thermostat setting on the boiler the louder the noise. The other problem we have following the service is that we cant put the front cover back on or the combustion products discharge safety device trips and the boiler shuts down but I guess this is not connected?