I did wonder if there is a water pipe running between the boiler and the upstairs rads over where I hear the hum. I suppose a plumber could tell me that? When I listen to the boiler close up, all I hear is the normal boiler sound...no pump running. I haven't tried switching the power off yet...I have more or less discounted an outside source as it is so centralised in my sitting room. I think a visit from a plumber may be required....