Posting as answer:
Did you try shutting off power to your house at the main breaker? That would rule out any AC-mains connected devices.
What about your toilet? The fill valve can make noises like this and it can be continuous if it's leaking.
The best way to eliminate transformer noise is to purchase a quality transformer.
The noise is because the laminated core is heavily loaded or overloaded and the plates are actually vibrating because of eddy currents. I can’t quite make out the size, but it looks like 24V, 20VA. I would bump it up to a 24V, 40VA unit that would be compatible with video ...
First, even though the exposed wires are low-voltage, likely 16 VAC, they should be covered for esthetics (if the transformer were accessible to children, not inside a closet, it is likely required). That said, the buzzing noise is likely being transmitted to the metal plate and wall, which make it much louder. A few things to reduce the buzz:
Remove the ...