Answer: Walla Walla
In the early days of broadcasting, producers realized that they could easily mimic the murmur of a crowd in one restaurant or another public space, in which the actors who are not currently involved in the dialogue repeat the word "walla" again and again.
With the advent of television programs where the audience could see the crowd, they occasionally switched to improvised conversations (so it does not seem like everyone in the restaurant strangely sings the same thing over and over again). Despite the departure from the use of the pat phrase, the term is firmly rooted in Hollywood, and actors who provide the background noise for the post-production effect placed over the video of the background extras are referred to as the "Walla group".
Particularly interesting about the phenomenon "walla walla" is that the phrases used by the actors are strongly regional and influenced by the native language of the region. American actors said "walla walla", British actors said "rhubarb rhubarb", German actors said "gaya gaya".