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Sounds like h-nmm , hh-aaaah , hn-hn , unkay , nyeah , ummum , uuh , um-hm-uh-hm , um and uh-huh occur frequently in American English conversation but have thus far escaped systematic study. This article reports a study of both the forms and functions of such tokens in a corpus of American English conversations. These sounds appear not to be lexical, in that they are productively generated rather than finite in number, and in that the sound–meaning mapping is compositional rather than arbitrary. This implies that English bears within it a small specialized sub-language which follows different rules from the language as a whole. The functions supported by this sub-language complement those of main-channel English; they include low-overhead control of turn-taking, negotiation of agreement, signaling of recognition and comprehension, management of interpersonal relations such as control and affiliation, and the expression of emotion, attitude, and affect.
Nigel Ward (Tue,) studied this question.