I have a baxi combi 80eco system recently fitted by previous owners. After the CH has been on for 30-60 mins loud noises sounding like a mixture of a whale or a horn lasting for 5- 10 secs happen 2-10 times per hour. It sounds like thay are coming from either upstairs or the attic. All radiators get hot all though the upstairs ones cool down quite a bit after an hour or so. Have bled all radiators with no success. Any advice appreciated.