Before you start to think you are completely doo-lally, do you have a mains water supply? Or even a pumped local supply? I ask, because the hum that I have been hearing on and off for years seems to be coming into the house through the water pipes. I suspect there is an electric pump running somewhere in the system, presumably at the water works, and the vibrations from this are being fed down the system and amplified by the walls of the house. The pipes,obviously, are secured by clips to the walls, so there is a very good path for the vibes to follow. Anyway, that is how I explain it to myself, otherwise I'd have torn the house down by now trying to find the source. I still don't really understand why everybody can't hear it. However, due to operating helicopters for many years before ear defenders became compulsory, I do have some frequency deafness and that may be a factor too.
Occasionally, the hum is more of a throb and sounds like a large capacity heavy industrial diesel engine running some way off and muffled. Then my water pipe theory starts to go out the window and I do wonder if there is tunneling going on deep down below my home. Specially as there are several large military bases not far away, two of which are known for their heavy security and somewhat secretive goings on. I live in the UK on the edge of Salisbury Plain.