Was on the train and was hearing a pulsing buzzing sound for a few seconds, thought should I be worried
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Flowing current generates a magnetic field, and the alternating current in your typical power delivery systems changes the direction of this current (and the magnetic field) 120 times a second.
This rapidly oscillating magnetic field can physically vibrate things and generate a buzzing sound.
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Transformers usually make a humming sound. Everytime the magnetic field changes, it can cause the transformer to vibrate. 60Hz is a low frequency hum, but higher frequency transformers give off a high pitch sound. At my job, only the younger guys can hear the whine of the higher frequency, it’s beyond the range of older people ears.
But other parts can also make sounds. A capacitor makes a sound when quickly charged or discharged as well.
Large transients, like turning on a high powered device, can cause large inrush currents into the tx, making it hum more loudly until the load reaches a more steady state.
“Buzzing” is a particular pattern of vibrations, and it’s not always caused by electricity. It can just as easily be caused by a mechanical part vibrating from being moved, rubbed against, etc.
Electricity doesn’t make any sound except when it’s physically moving something to cause it to vibrate, or when it heats up air to make it expand (thunder!). You frequently hear electrical parts called transformers buzz because the electricity going in is alternating current, meaning it flows back and forth, and that causes a magnet inside the transformer to jiggle (at 50 – 60 Hz, the frequency that they power cycles). The same thing happens in the “ballast” portion of fluorescent light fixtures for exactly the same reason.
The sort of electrical buzz you hear from transformers and lights is so common, that movie cameras and software used for editing movie sound have special filters that can be used to filter it out. Movie and TV sets tend to have lots of lights and equipment that generates that buzz sound, and sometimes the microphones pick it up.
I’ll add for fun fact that since all of our electricity (AC) has this “hum” happening in the background, that signal would be present in guitar pick ups and when run through an amplified you’d hear that “hum” happening. People didn’t love this so they developed a new pick up that loop back on themselves, essentially cancelled out the hum signal.
These pick ups are called “humbuckers” for this reason.
Corona discharge on high voltage (transmission) power lines can be audible under certain circumstances. This is a separate phenomenon from the “humming” produced by transformers. Corona is caused by the ionization of air, and it will sound like static.
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