Why is this a good way to describe the noises?

Are you referring to the Owl Service?

The Language itself becomes an important theme for the book, mostly highlighting the different in the social status between the English character's, Gwyn and the Welsh character's. Over all, the choice of words for each character and the book entire is that of a simple, straightforward and direct use of words without long explanations or adjectives. This allows the book to be presented as a 'children's book' and the meanings understood fairly clearly and immediately.

An interesting side note however worthy of inclusion here is the use of seventies British nomenclature which has largely gone out of fashion nowadays (and reflects the times that the book was written, and the antagonisms between the two cultures at the time).