Have recently had a new Baxi 105 instant fitted in my Kitchen. After about a week it started to make a howling noise while the CH was on. Got the heat team engineer out and he replaced the heat exchanger. All seemed well then a few days later it started again. Out he came again, replaced the exchanger again. Few days passed, started again, but more of a high pitched humming noise, can be heard all over bungalow. Engineer back with installer - changed diverter valve, put some stuff in one of rads with silicone gun. Still got a constant humming/ droning pitch noise while CH on. Getting fed up now. Both engineer and installer think I am imagining it. Never had this with old boiler. Anybody out there have any ideas or suggestions. Have thought about boxing it into an MDF cupboard, or moving into loft. Thanks