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Gaz cheers for reply but can`t see it being low pressure setting as it`s doing it on full gas with all trvs on full with no heat in system from cold. Jason boiler is 2 months old and done from day one what are you thinking re flow switch.The boiler is a mates who is too busy fitting them to call baxi out so i said i would ask on here but looks like he will have too. we had one do exact same last year baxi guy fixed it in 15 mins according to customer but didn`t say what it was. Chris /kev cheers for that thread excact symptons except its a drone instead of whistle. Doitall BG may have all the answers but i wouldn`t know cause i don`t work for them but i suspect they have more answers than you would have.