When humans are happy they laugh or smile, and cats? When they are happy they purr.
Cats purr is happening when the cat's brain, vibrating the muscles in the voice box, which stops and continues the flow of the air. This mechanism works during inhalation and exhalation and it makes the impression that the purr is continuously.
Cats purr around twenty six cycles per second.
(Cats may also purr when they are upset or startled.)