Gibbons are part of the Lesser Apes along with the Siamang. It separates them from the Great Apes (humans, chimpanzees, bonobos, orangutans and gorillas), but they are still part of the Hominoidea, or Apes.
Both in the wild and in captivity, noise is their best option. A lot of primates will display, beating their chest (like in gorillas), screaming (like in the chimpanzees) and hooting (like in the...