Unfavourable weather conditions in particular, such as rain, hoar frost or wet snow, can cause local electrical discharge in power lines. In electrical engineering, this process is known as corona discharge. The phenomenon can produce noises described as crackling or humming.
In Switzerland we have an emissions limit of 55 decibels in residential areas (45 decibels at night), which must be adhered to by law. The noise pollution from a busy street is over 80 decibels. Where necessary, Swissgrid employs all technical means to limit the corona effect. Corona noises are not present in underground lines.
The following movies show the sound intensity of high voltage power lines compared to more common ambient noise: