Several different types of technology are used to produce common household light bulbs, notably incandescent and fluorescent lights. Incandescent lights use a current and a filament to produce light through resistance, while fluorescent bulbs use a gas and a phosphor coating to produce light by energising the atoms of the coating. Both bulbs can flicker or make small noises. (LED lights are not subject to the same problems.)
Fluorescent lights operate according the cycle of electricity that constantly flows through them. Older fluorescent lights have 60-hertz systems that slow down this cycle until humans can notice it. The cycle looks like a constant, dull flicker, and is completely normal. Fluorescent lights are now made with 120-hertz systems that cycle too fast to be noticeable to the human eye.
Fluorescent lights require ballasts, or small electrical devices that manage voltage and flow to the bulb, making sure the current is uniform. Older ballasts were magnetic in nature, using an iron core that took a little while to start up and channel the current. These ballasts tended to hum or buzz, especially when first lighting up. Newer electrical ballasts should not make any sound at all.
Incandescent lights operate on simpler principles than fluorescent lights, but they can still be subject to flickering if their filaments come loose. The filament is extremely durable and can resist the electrical current that passes through it, so that energy is built up and released as light. When the filament is loose, the electricity may not be able to complete the circuit all the way through the bulb, causing the light to flicker on and off. The bulb must be replaced to solve the problem.
If the filament is used in a dimming application where the flow to the filament can be adjusted to create a dimmer or brighter light, the filament can sometimes vibrate, creating a humming sound as it struggles to adjust to a new setting. This vibration tends to stop when the dimmer is adjusted to a different setting. A loose filament may also make this sound in a normal bulb.
If all lights in a section of the house are flickering, or if the flickering is continuous, it may be caused by an arcing problem. Arcing is caused by small gaps in electrical wires in the walls where the current must make a small jump from one part of the wire to another. This can be a dangerous fire hazard, and homeowners should call an electrician if this seems to be the problem.