Switch off your electricity at the mains. If that stops the noise you have something to work with. Switch the leccy back on and visit each individual appliance and switch it off until you find the culprit. It may in fact be your meter itself. They do sometimes make a noise and need to be replaced.
We tracked our "noise" down to a freezer that was vibrating against a wall.
When a similar noise was detected some twentyfour months later I repeated the exercise and found that the stereo unit was on standby (because that is its default mode and to switch it off completely means crawling behind a bookcase to switch it off at the wall socket) and that one of the speakers was giving out a permanent hum of "feedback", presumably picking up the small amount of power being used whilst on standby. Moved the speaker and the problem ceased.