The happy sound a relaxed cat makes is a purr. A purr is low and fluttery, and you can feel it vibrating in the cat's body.
Your pet cat purrs, and so do some of her larger relatives, like the puma and the bobcat, as well as the cat-like animal called a genet, raccoons, badgers, tapirs, and even guinea pigs. Scientists aren't entirely sure how a cat purrs, but they do know that felines can either purr or roar, but not both (lions, for example, don't purr). They also suspect that a purr evolved as communication between mother cats and nursing kittens.