Mystery Hum

I am a light sleeper, so I am happy that my bedroom is way at the back of the house away from the street noise. But lately I've noticed a hum--an electronic hum that is constantly on in my room. I do not hear it in the sitting room or the back yard (although there are lots of other noises in the back yard competing for attention). It sounds like a refrigerator hum, but our refrigerator does not make that hum. It sounds electronic, like maybe from a tv, but I unplugged everything in my bedroom and sitting room (even the cable box) and the hum was still there. I thought it was the watering system on the side of the house, but I put my ear really close to it and there's no hum. I thought it was the electrical converter box at the back of the property, but I cannot discern the hum outside at all. Dad suggested that my bed frame is channeling some sound from within the house, maybe the front of the house and I'm tempted to shut the power in the whole house down to find out (but I don't know where that switch is or if it's even a switch). Of course the only time this hum bothers me is in the middle of the night when it is highly inconvenient to get up and start playing with the power in the house. There has to be a reasonable explaination. I must destroy that hum.