system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What's exactly the low humming sound that you can hear near high-voltage areas, like electrical substations?
gpt
What you are hearing is the vibrations of various metallic components, caused by the changes in the magnetic field of the alternating current carried by the power lines. 

Because the current is alternating at a certain frequency (60Hz in the US, 50Hz elsewhere) that frequency is carried over into vibrations, so all humming installations hum at the same frequency / pitch.