A purr's a sound made by all species of felids and's a part of cat communication. It varies between cats (for example by loudness and tone), and from species to species, but can be characterized as a tonal buzzing. Domestic cats purr in a frequency of 25 to 150 vibrations per second. Although purring's commonly associated with felids, other animals such as raccoons purr. Other purring animals are: Guinea pigs in heat, rabbits, squirrels, ring-tailed lemurs, elephants while eating, and gorillas… (More on Purr)