Some of these problems directly impact the audio and/or picture quality. Let's examine each type of powerline disturbance below. Please understand that a full discussion of each of these issues would require a large chapter in a book -- these definitions merely scratch the surface.

In order to fix a problem, it helps to define the problem. Some of the problems are electrical only, but manifest themselves in the audio or video circuitry. Sometimes a problem actually originates in the audio circuitry itself.

Some percentage, perhaps 5% to 25% above of the "stated", "nominal", or "specified voltage".

Brownouts are the opposite of the overvoltage condition, above. The original issue of brownouts was when in large cities, heavy current demand for air conditioners in the summer put such a strain on the power grid that the voltage sagged, or went down. Sometimes this happens TO the power companies, and sometimes this is caused BY the power companies. (i.e. controlled or rolling brownouts) The end result is the same: less voltage is available and THEREFORE lower current is available.

Undervoltage condition caused by the powerline source being too high an impedance to supply the necessary current demand; could be milliseconds (such as from a motor start capacitor drawing inrush current ) or many seconds (any continual appliance, such as an electric iron, clothes dryer, heater, etc. This is more often than not caused by too small a wire used in an installation or by Aluminum wire used in place of copper. (Aluminum wire is cheaper, but has higher resistance than copper.)

The sine wave simply stops for a few milliseconds, or perhaps as long as a few cycles. This might be caused by a lightning protection changeover relay device somewhere upstream in the power grid, or, it could even be caused by an intermittent connection, and although that would be a rather rare condition, it does happen.

Higher voltage occurences which ride on top of the sinewave; perhaps up to 750V or so. A spike might be considered the shortest duration, perhaps a few microseconds to a few milliseconds. A surge might be from a few milliseconds to a few seconds. And a peak condition might last from a few seconds to a few minutes.

Perhaps 500 to 5000V lasting from hundreds of nanoseconds to hundreds of microseconds. It is these higher voltage spikes which burn out equipment, often in mysterious ways. Typically these high voltage spikes are caused by lightning somewhere on the grid, from a few feet to a few miles away

The opposite of peaks: V-shaped cutouts in the sine wave as seen on an oscilloscope

Power Factor is the phase relationship between the VOLTAGE condition of the line and the CURRENT condition of the line. A totally resistive load (i.e. a filament lightbulb) has "unity" power factor. A load that has a CAPACITIVE or INDUCTIVE component REACTS with the sinewave; therefore this is said to be a "reactive" load. This causes a harmonic imbalance of the sinewave, resulting in Harmonic Distortion, explained below. One goal of load balancing for example on a 240 Volt circuit is to even out the different power factors so as to NOT introduce either a voltage imbalance or a harmonic imbalance on the power line.

Odd and/or even order distortion of the 60 Hz sinewave causes mechanical humming in transformers. The waveform, instead of being a 'pure' 60 Hz sinewave, contains some combination of even (120, 240, 360, 480 Hz) or odd (180, 300, 420, 540 Hz) harmonics.

In European areas were there is a 50 Hz sinewave the EVEN harmonics would be 100, 200, 300 Hz; and the ODD order components 150, 250, 350 Hz.

Another mechanical 'humming' sound emanating from a transformer: the transformer windings are magnetically saturated, often because the voltage is too high. This saturation starts turning the sine waves into square waves. Square waves, by nature and definition, are composed of a series of odd harmonics. These harmonics mechanically vibrate the transformer and its mounting apparatus, and that is what you hear 'mechanically'. In the instance of a subwoofer, you might think the subwoofer [speaker] is making the noise, but actually it's the transformer inside that is making the noise and you are hearing this transformer noise through the paper or cone material of the speaker. This noise is often a mix of both 60 Hz and 120 Hz, and some of both of their odd harmonics.

RF frequencies (radio frequencies, i.e. from an AM radio station, TV station, CB radio, Ham / Taxi / Police / industrial walkie-talkie radio, etc etc) amplitude modulating the sine wave envelope. This is commonly referred to as "picking up RF". See also rectification, below.

Noise common to BOTH sides of the line (sometimes a problem when operating from 240 Volts)

Noise showing up on one side of the line measured to ground. Therefore in the case of a 240 V line the noise would be only or mostly on ONE side or leg, i.e one of the 120 V "legs".

Ground noise or currents caused by bad, loose, or high impedance ground connections; can also be caused by rectification in the grounding wiring, where corrosion forms between copper and aluminum connections.

Usually caused by dissimilar metals such as aluminum / copper junctions which then rectify the RF in the air and add the resulting modulation voltage to the power line.

Where some digital "audio" or "power supply" circuit feeds its switching noise back into the power line and the noise then shows up in another piece of equipment.

Sonic or ultrasonic "whine" from a poorly designed switching power supply leaking back into the power line. Expect a whole new generation of problems with so-called "switching" audio amplifiers - especially as they get old.

Caused by loose or dirty mechanical connection; even caused by such seemingly innocent devices as pushing a wire into a captive wire slot in a switch instead of properly screwing the wire down securely under the connection screw plate.

May also come from relays or motor brushes. If a switch was opened or closed at EXACTLY the zero crossing point of a sine wave, since there is no voltage present there is no current flowing. Since this is not a very likely occurrence, most of the time when either a relay opens or a person opens a switch the waveform is at some other spot than zero, and therefore there IS current flowing. The current attempts to continue to flow across the airgap, causing arcing. Arcing then oxidizes, pits, and eventually wears out the metal mating surfaces.

Some [better] circuits use relays filled with nitrogen which will not allow an oxidizing arc to form... since there's no oxygen inside the relay.

Often amplified noise from the previous circuit; this often implies improper gain staging or matching, but may be a defective component as well, acting as a noise 'generator'.

A true ground loop! A true ground loop is ONLY a 60 Hz sine wave component !
In the UK and other 50 Hz countries, it's 50 Hz.

Quite likely a power supply filter problem; however most people "think" that 120 Hz hum is a ground loop. It isn't!

Perhaps a bad capacitor, noisy transistor junction, or IC / OpAmp going bad or latching up.

Buzzing is typically 120 hz that is clipped, (or where the filtering circuitry in the power supply has faulted) generating a series of ODD harmonics, i.e. a mix of 120, 360, 600, 840, 1080 Hz, distinctly measureable by specific test equipment, such as a spectrum analyzer. Since it is clipped, it has turned into square waves; square waves by definition are odd-order harmonics. So a true buzz has a very recognizable sound. Please note that Hum and Buzz are completely different phenomena.

Hearing the demodulated audio from AM, FM, TV, ham, shortwave, Marine, Taxi, CB, etc etc. This is NOT the same as leakage, where one signal leaks into another undesired area usually because solid state "switches" are used rather than mechanical ones.

A sound that seems ike a combination of a buzz and noise, typically caused by "office" fluorescent lights and their starter circuitry.

This is the noise you hear from "dimmers" where the buzzing changes pitch as the dimmer knob is rotated; you are hearing the on/off duty cycle of the dimmer change.

Any overvoltage condition such as a spike will usually produce a tick in an audio circuit. Only the very best power supplies will really filter this out, and even they have no control of this if the tick is riding on an audio signal which is properly going through an amplifier. This could also be caused by a refrigerator motor or washing machine motor (on the same circuit) starting up. The higher impedance the entire chain of wiring is, the more likely this will happen and be noticed.

59.94 hz beating against 60 Hz; often caused by ground differentials between a feed from a "TV cable company" and the local house or studio ground; perhaps as high as 65 volts differential. This manifests itself as a buzzing sound, cyclical in nature, sort of repeating every 14 seconds. Once this sound is heard and memorized it is VERY recognizable.

Fabulous white papers / application notes on the Jensen Transformers site:
http://www.jensen-transformers.com/application-notes/

Another treatise, originally courtesy Middle Atlantic morphed into Legrand company's white papers section... HERE  (superb, and a clickable table of contents)

Surgex has morphed into ESP (Electronic Systems Protection) which has morphed into AMETEK, and their technical library is here: https://www.ametekesp.com/resources/white-papers