ahem vs utterance

ahem and utterance both are nouns.

Word	Noun	Adjective	Verb	Adverb
ahem	Yes	No	No	No
utterance	Yes	No	No	No

As nouns, utterance is a hypernym of ahem; that is, utterance is a word with a broader meaning than ahem:

ahem: the utterance of a sound similar to clearing the throat; intended to get attention, express hesitancy, fill a pause, hide embarrassment, warn a friend, etc.
utterance: the use of uttered sounds for auditory communication

Other hypernyms of ahem include vocalization.

ahem (noun)	utterance (noun)
the utterance of a sound similar to clearing the throat; intended to get attention, express hesitancy, fill a pause, hide embarrassment, warn a friend, etc.	the use of uttered sounds for auditory communication