Audio frequency track circuits are used so that there is no need for insulated rail joints (IRJ). The noise comes from the track side units (older units were a cream colour, later units are a light grey colour).
The noise is present whenever there is no train on the track circuit concerned.
The cause is the inductors in the track side units vibrating due to the pulsing electrical current flowing through them. As the current changes, so does the magnetic field, and hence this force causes just enough movement to cause a vibration.
Because the sound is in the range of human ears, yes, you can hear it directly. And obviously microphones can pick it up as well.
With the older type, each track circuit uses a single constant frequency. But the more modern later type use two frequencies that alternate, just like some sirens.
I hear a noise through my camera at sharnbrook junction. A warble electrical noise that sounds like interference from somthing nearby. I would imagine where 3rd rail is used this warble is more evident due to the amount of transformers and heavy duty cabling being used.
The third rail supply equipment will vibrate at the supply frequency. For rectified DC, the frequency is doubled compared to the AC input to the system.
Large mains transformers can often be heard vibrating at 50Hz. Substations can often be heard for example. As can some older style fluorescent light fittings that use a magnetic choke (inductor).
But none of these will warble like a siren.
In this screen grab from the video in post 7, you can see some of the track side units.