Growling or growl is a low, guttural vocalization produced by predatory animals as a warning to others, as a sign of aggression, or to express anger. Low or dull rumbling noises may also be emitted by human beings when discontent with something or angry. The human sound is often termed "groaning".
Grrr /ˈɡɜrːː/ is an onomatopoeic word which imitates the growling sound of predatory animals, and is often used with other related meanings. It is one of the rare pronounceable words of the English language that consists solely of consonants. Its most simple use is by children imitating animals. An example would be: "Daddy! Look at me! I'm a polar bear! Grrr!" This word is also widely used in various titles to express growling.