A "substation" is a transformer. In is used to change the line voltage. because high voltages are used to transfer power over long distances (as it is more efficient). However, running your house on 11kV would be dangerous, as high voltage can "jump" or "arc" large distances.
So, these substations take the high voltage low current power and use a transformer to reduce that to a lower voltage but higher current.
The system runs on an alternating voltage at 50hz (50 cycles per second) and this sinusoidally varying voltage is used to create a magnetic field within the primary windings of the transformer. That field is then re-captured by the secondard windings and turned back into electricity (at a lower voltage in this case).
These transformers hum because the magnetic field causes parts of the transformer to vibrate (at 50Hz), just like a loudspeaker coil does. To maximise efficiency a typical transformer will be over 99% efficient, in effect much less than 1% of the magnetic field will "escape" and leak out.
Transformers tend to get noisier as they age, as when new all the windings and laminations in the core are tightly packed and well supported, but as they vibrate over the years they tend to get a bit looser and hum a bit more.
Crucially, magnetic field strength reduces with the square of the distance from the emitter, so if you are more than a foot or so from the transformer, the magnetic field strength leaking from it will be less than that created by the earths natural magnetic field!
In summary, unless it arcs, overheats then explodes, you are quite safe!