Any unwanted sound or distortion appearing at the loudspeaker of a radio receiver. A portion of the noise is due to interference from various sources: disturbances in the atmosphere or discharges in the ionosphere (atmospheric noise), extraterrestrial sources such as the sun (galactic noise) and signals from other radio transmitters and other man-made sources. The remaining portion of the noise is an inherent property of the electronic circuits and devices and is caused by the random motions of electrons in the circuits.

A very common form of noise is mains hum, which is caused by harmonics of the mains frequency being detected and amplified by the radio receiver, and results in a humming noise. It tends to worsen as the components in the receiver age. See also interference; noise.