from reddit by Nebonit
The sound you’re likely hearing is partial discharge or corona discharge, not the sound of the frequency of the electricity (unless you’re listening to a transformer). It’s the noise that air makes as electricity jumps through it, basically baby lightning, for power companies this is a problem for polymer and oil based insulation (ceramics don’t mind) as it degrades their ability to resist the voltage. You can’t hear this on low voltage since there isn’t enough ‘pressure’ on the electricity, you could however hear the frequency of the electricity in something like a microwave transformer or a electric motor that is stalled. The higher the voltage, more humid and if there is a sharp point can all make it louder, so have a listen to it on a humid night and you might even see it.
from reddit by Manodactyl
Follow up question, why do I hear the insulators buzz when it’s cold and humid (generally at night), but don’t hear them when it’s warm out (during the day)
from reddit by mattskee
At night the relative humidity is usually higher because of the temperature drop, and you may also have some slight condensation on the insulators. The additional water in the air, and possible water on the insulators, reduces how good the insulators insulate so you get more leakage of electrical current making that sound. On a really foggy night I have even seen insulators periodically flash over, which probably clears them of their condensed water and they continue working.