Purring

A noise that a domestic cat or possibly other creatures make to indicate well-being. A cat will also sometimes purr when it is uneasy or nervous, to seek reassurance.

Science still does not know how cats purr, but one theory holds it involves cats false vocal cords, the other involving blood hitting the aorta. A cat might try to meow while purring; this usually results in no sound being produced at all and the purring momentarily ceases.