Hi i was asked to take a look at a baxi 80e loosing pressure so checked the regular items diverter, prv, expansion vessel cleaned out pipe to expansion vessel but a couple of days later the customer mentioned a water hammer noise when the heating was on went back made sure the radiators had no air in checked the air release valve in boiler was open and the noise did die down but is still there slightly anyone know what would cause this the system is on mini bore pipe could it be a bit of an air lock?