My house literally hums all the time. It started when I had a new boiler fitted (not a combi - I have a hot water tank in airing cupboard and water tank in the loft). The hum sound is like when the central heating is on but is there even when the central heating is off - and does get very loud sometimes. I have had the boiler serviced and they said it was okay. I have turned gas, water and electric off and the hum is still there. Any ideas please?