The electricity that comes from power plants uses alternating current (AC), so named because the current changes direction, or alternates, many times per second.
The number of times per second the current alternates depends on countries’ standards. AC power usually alternates at 50-60 hertz, or 50-60 times per second.
The hum you hear is usually about twice the frequency of the AC power being used, when electricity hums at 100-120 hertz.
It usually is an electromagnetic device called a transformer, which is used to decrease the voltage of power, vibrating that creates the hum.
The reason power lines themselves might hum is due to corona discharge – discharge of energy that happens when the electrical field around the power lines is greater than what is needed to start a flow of electric current from the power line to the surrounding air. Most modern power lines are designed to avoid this problem.