Hmm...

A speech disfluency,

also spelled speech dysfluency,

is any of various breaks,

irregularities,

or non-lexical vocables

which occur within the flow

of otherwise fluent speech.

... "false starts",

i.e. words and sentences

that are cut off mid-utterance;

phrases that are restarted or repeated,

and repeated syllables; "fillers",

i.e. grunts, and non-lexical

or semiarticulate utterances

such as uh, erm, um, and hmm,

and, in English, well,

so, I mean, and like;

and "repaired" utterances,

i.e. instances of speakers correcting

their own slips of the tongue

or mispronunciations

(before anyone else gets a chance to).

Hmm is an exclamation

(an emphatic interjection)

typically used to express reflection,

uncertainty,

thoughtful absorption,

or hesitation.

Hmm is technically categorized

as an interjection,

like um, huh, ouch, erm, and wow.

The first h-sound is

a mimic for breathing out,

and the second m-sound,

since the mouth is closed,

is representing that the person

is not currently sure what to say...

The variety of tones, pitches,

and lengths used add nuances in meaning.