Intermittent and continuous noise
Noise falls into two broad classes, intermittent and continuous. The problems, and their treatment, are dealt with on separate pages under this heading, because they require quite different approaches.
Intermittent noise is typically in the form of clicks and crackle. The origin is usually scratches and/or dust particles in the grooves, electrical power switching noises, or imperfections in the recording medium. In between these glitches, the sound may be of good quality, so the usual kinds of filtering does more damage than good.
Continuous noise is a common problem with recordings captured from a diversity of sources, for example from magnetic tape (cassettes, etc.), as well as from older gramophone records. The most common forms of such noise are Hiss, particularly tape hiss and hiss from old shellac records, Rumble, usually originating in the turntable mechanism, and Hum coming from inadequate shielding from main frequency supply at 50 or 60Hz.