A purr is a sound made by all species of felids and is 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 is 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 gorilla… (More on Purr)