The breaker box, also known as the circuit box, is the primary device for protecting a home from electrical problems. The home is divided into "circuits" where only a certain amount of electricity is allowed to be used at one time.
Often these circuits include all the electrical outlets in one room or those close together. Some outlets for power-hungry appliances such as an electric stove or heat pump have their own circuit, with no other electricity user able to draw power down that line.
Tripping a Breaker
If something goes wrong on a circuit, such as too much power being asked for by one or more appliances or a feedback from wires that have become tangled and are shorting out, the breaker for that circuit will "trip" and cut off all power to that circuit, protecting it from a potential fire from overheated electrical wiring. If it is a momentary overload, the switch at the breaker box for that circuit can be manually reset with no harm.
Occasionally there may a problem within the breaker box that prevents it from tripping the circuit. The box then makes a loud humming or buzzing noise to alert you to a potentially hazardous situation.
Because the heat pump operates on a much stronger 220V circuit than standard 120V household appliances, the chance of a fire happening is much higher. Any time the box makes a noise, a repairman should be immediately called in.
It is common for lights to momentarily flicker when a unit as powerful as a heat pump activates. However, if the lights continue to flicker, this means that there are problems back at the breaker box.
Somewhere within the box wiring, the dedicated circuit is touching on other circuits and interfering with them as the power is drawn through the wire. This disrupts the power to the lights, which causes the flickering.
With both flickering lights and a buzzing noise, it is far safer to keep the heat pump unplugged and not operating until the breaker box is checked and repaired by a qualified electrician.