Why do overhead electric power lines make noise?

The 'buzzing' noise associated with power lines is due to the chemical breakdown of the air due to intense electric fields. This is at its greatest in the vicinity of sharp-edged conducting components which occur near the insulators which support the power lines This effect also accounts for the hazy blue discharge often seen at night. The effect is at its greatest during damp conditions.

On the other hand, the 'humming' heard from Transformers is due to the 50- or 60-Hz electromagnetic forces on the device's laminated core.