Scientists have been studying the sound produced by male Koalas.
They make a deep grumbling sound when they want to communicate with one another.
The grumbles come from a special sound-producing organ not seen in any other land mammal.
While other mammals, including humans have one set of vocal folds, Koalas have two which is why the small creature can make such a deep sound.
Watch the clip to hear them grumble!