What is the rapid sequence of soft vocalizations used by gorillas during friendly contact called?

The rapid sequence of soft vocalizations gorillas use during friendly contact is called the gorilla belch vocalization.

Gorilla belch vocalizations are gentle, pulsed sounds associated with relaxed social situations, including feeding and proximity to familiar group members. Despite the name, the term refers to a communicative sound rather than ordinary digestive belching. Researchers have described these vocalizations as part of the rich contact system of western lowland gorillas.

Gorillas communicate through many channels. They use grunts, barks, screams, hoots, chest beating, facial expressions, posture, touch, and scent. A sound’s meaning depends heavily on context, such as whether an individual is calm, threatened, soliciting attention, or interacting with a group member.

The term is easy to confuse with “vocalization” used as a broad category. “Gorilla belch vocalization” is the specific label for this soft, socially associated sound, not a name for every noise made by a gorilla.