In the English-speaking world, and probably also in many other cultures, you can signal imbecility or infancy (on the part of the first, second, or a third person) by keeping your mouth closed, or nearly so, and then vocalizing while flicking your finger up and down over your protruding lips. The vocalization is usually a fairly neutral, unchanging pitch and volume.
I recently watched a movie where a character did this, and the SDH subtitles captioned this as "[ululates]". "Ululates" struck me as being the wrong word, since I've only ever heard it used to describe a loud, high-pitched wail, usually with rhythmic and rapid movements of the inner mouth parts. Reference works such as the Oxford English Dictionary, Wiktionary, Wikipedia, and Merriam-Webster generally agree with my conception of the term. Some of them refer specifically to the tongue and/or uvula being the manner of articulation, some mention that the sound is used to express "joy" and/or "lamentation", and others refer to the sound in more general terms as simply a "howl" or a "wail". None of them mention digital manipulation of the lips, and none of them indicate that the sound is used to signal imbecility.
Does English have a specific word or a set phrase to denote the particular sound/gesture I'm describing?