why do electric lines hum and crackle?

Answer 1

two phenomena make those sound.

a 100Hz hum (120Hz for USA) is done by electrical current passing through lines. The current creates a magnetic field around each of the lines and because the current is alternative also the magnetic field oscillate. This means that the lines are attracting and repel each other thus they vibrate and make that hum.

the crackle is know as Corrona effect = micro voltaic arcs on insulators surface, too small to be visible.
voltaic arcs have a very high temperature and heat the air which expands very quickly and makes the noise. (e.g. the thunder from a lighting)

Answer 2

Electricity