Sounds like either literally wires rattling (or some loose bits) or the metal expanding and contracting with changing temperatures throughout the day. Most noticeable at night when the temperature changes fastest and the ambient sounds are quietest.
edit: Read a bit more in the comments–you’re talking about electrical hum–sounds like cicadas! [https://www.scienceabc.com/innovation/why-do-power-lines-produce-a-buzzing-sound.html](https://www.scienceabc.com/innovation/why-do-power-lines-produce-a-buzzing-sound.html)
Basically it’s the sound of electricity arcing through the air around the wires.
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