Many people associate this kind of vocalization to cats. A purr is a low buzzing sound a cat makes when it is contеnt. The sound may sometimes be very hard to hear, though it is quite distinctive. Sometimes, they are fairly loud.
The exact mechanism by which the purr is generated is still to be identified but from earliest kittenhood the cat's purr gains attention. In kittenhood, together with their front paws, they knead their mothers' belly to find to source of milk. The mother responds by inducing the milk let-down reflex, enabling the kittens to suckle. In adulthood, cats can make this sound when happy, contented and relaxed. However, in some cases, extreme distress, hurt or sad, cats tend to purr to alleviate the tension.