Far too many unknowns here to make any diagnosis or suggested cure.
How big are the transformers, what is their power rating?
There are 4 transformers that are 12 volts DC each and range from 300 to 500 milliamps.
What is the electronic device which is humming?
The device is called a "Listen Up" in the United States.
In what way does it hum?
Over my headphones.
Does it have a loudspeaker which hums, or does it just make a noise?
Headphones.
What is the power supply to this place? Is it on the end of a long overloaded cable?
I have a "DigiVue" video-camera card. It has a four camera capacity. I have my computer in my den which is in the rear most part of my house. All four cameras plus the "Listen Up" is located in the front of my house to watch the street as well as listen. My "Listen Up" is attached to a TRS stereo plug and uses the line intended for sound as of course there are three lines, one for the volts, one for the camera and one for the sound. Where the sound, volt and camera lines terminate in my den is where I have attached what is known as an "isolation transfomer" to the sound line to kill the aweful "DC hum" I've read it once described. On the other side of the "isolation transformer" I have attached by solder a TRS plug to plug into the "MICROPHONE" female plug which is on my "Xtreme" sound card. Everything works very well except for the hum.
JimB