Can anyone offer help please ? I have a Swiftflow 80 and when been on for about 30 minutes and warm, there is a persistent and very irritating tapping noise (which I imagine is expansion and contraction caused by hot and cold air) from the flue. I have a maintenance contract with a very large Gas Company and they have been out three times now, giving me reasons such as \\\"that\\\'s a design fault\\\" Through to the hole which the flue enters to the outside wall hasn\\\'t been drilled straight and this causes problems. They appear to be very reluctant to chnage the flue. Any ideas ?