Why do high voltage power lines hum or crackle?

Answer
The hum is being made by Corona Discharge.  This basically happens when the electrons in the wire want to go to the air around it because it has such a high electromagnetic field.  What’s going on is that the electric field gets so strong in the air around a wire that the electrons start to get pulled off their atoms and into the air.  Usually air is an insulator around the wire but in this case it ionizes and becomes conductive plasma!  So the noise happens when the voltage is too high for the air around it to handle before it breaks down and becomes a conductor.

Facts:

• It is loudest at sharp points, corners, bolts, or any surface irregularities. 
• It can be affected by the condition of the temperature or air such as humidity, air density, wind, fog, rain, snow because they all change the electrical resistance of the air around the wire.
• Dry air changes its mind about being an insulator and becomes a conductor at about 10 thousand volts per inch.
• It’s not just loud but it actually wastes power, like a leaky pipe.
• The voltage on distribution lines is low enough that this doesn’t happen, only transmission lines.
• It happens all the time in nature.  You can see it during thunderstorms as a faint glow around sharp rocks, trees or bushes.
• Sometimes you can even see it at night or smell the ozone it creates.

A high pitch alien whine sound is caused by something else called Aeolian vibration.

Geek summary:  In other words the audible noise emitted from high-voltage lines is caused by the discharge of energy that occurs when the electrical field strength on the conductor surface is greater than the 'breakdown strength' (the field intensity necessary to start a flow of electric current) of the air surrounding the conductor.
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