feedback
The howling or sustained tone produced when amplified sound from a speaker re-enters a microphone or guitar pickup.
In Depth
Acoustic feedback occurs when sound from a loudspeaker is picked up and re-amplified in a loop, creating a sustained or escalating tone. Once an unwanted nuisance, guitarists like Jimi Hendrix and Pete Townshend turned feedback into a musical tool, using it for sustain, harmonic overtones, and raw sonic expression. Controlled feedback is now a standard technique in rock, noise, and experimental music.
The opening chord of the Beatles' I Feel Fine (1964) is generally credited as the first deliberate use of guitar feedback on a pop record.