Can you turn power to the whole house and see if the noise goes away?
I am always surprised by how quiet our house is when there is a power failure.
I would not be surprised to find that what you are hearing is the cumulative effect of modern electronics. Certain LEDs in our house hum (but I can I am only aware of it when I am close the the LED) certain cell phone chargers have a wining noise to them (again only hear as a distinct noise nearby), one computer power brick makes an annoying whine (I notice it when things are quiet and I cannot sleep if that computer is in the room), the wireless charger for my phone has a small fan (a little bit of a white noise), the ceiling fan has a dull noise from what I assume to be the bearing, the 12VAC converter for the doorbell has a slight coil whine, one of my GFIC outlets has a low buzz, and I am sure I have ID'ed a few other things.
I am sure there are tons of things sitting there with power draw that could be emitting a little noise, think anything with a little glowing LED even when it is "off", waits for a remote control signal, has a touch input without physical power switch, or heck anything that converts 120/240vac to 12vac or DC.
Most of those are not independently discernible beyond a couple inches to, a most, feet, but could add up to a low background of white noise/hum.