What is it called when one expresses "Yes", "No", or "I Don't Know" without using words - just humming their response? You know, when someone responds with a low note than a high note for Yes or "Ah-huh!"; a low note and a slightly lower note for No or "Nuh-uh"; or a low note, a high note, and a low note for I don't know.
In a sentence, it's often called a grunt or murmur of assent.
A related question is How to describe making sounds with your throat?
I believe the word falls into the category of paralanguage. http://www.dictionary.com/browse/paralanguage