Rhubarb! Rhubarb! Rhubarb!

A phrase much used in the Goon show but apparently extras in films and TV shows are often ordered to repeat "Watermelon cantaloupe" or "Rhubarb rhubarb" over and over in order to imitate the murmur of a crowd in the background of the scene. The mouth movements to say these phrases are sufficient to give the illusion of real speech.