Ostriches make two kinds of noise: the males make a low, deep noise called a boom. They do this by inflating their neck but keeping their beaks shut. Males 'boom' as a mating technique, marking their territory. Female ostriches, on the other hand, make a hissing noise with their beaks open.
It is thought that birds descended from dinosaurs. Some scientists study vocalization in different animals. They think that dinosaurs may have made closed-mouthed sounds like ostriches too, although much louder.