 One hypothesis, supported by electromyographic studies, is that cats produce the purring noise by using the vocal folds and/or the muscles of the larynx to alternately dilate and constrict the glottis rapidly, causing air vibrations during inhalation and exhalation.Combined with the steady inhalation and exhalation as the cat breathes, a purring noise is produced with strong harmonics.Purring is sometimes accompanied by other sounds, though this varies between individuals.