Why humming sound occur in extra high tension transmission lines?

Why humming sound occur in extra high tension transmission lines?
Awaygood Answered First
The noise is better described as 'buzzing', rather than 'humming', and it is caused by the breakdown of the air's insulating properties, due to stress caused by electric fields set up in the vicinity of where the conductors are supported from insulators. The noise is accompanied by a pale blue discharge which becomes visible after dark.