Why do high tension wires produce a humming sound?

Air surrounding the conductors of a high voltage overhead transmission line is normally an insulator. But at extra high voltages (66,000 volts and above), the air in close proximity to the conductor tends to break down under the voltage stress, along the length of the conductor and itself becomes a conductor. This is known as corona effect. Corona appears in two forms, visible corona as a bluish violet light and audible corona as a hum. Audible corona is common. Visual corona requires a higher voltage stress.