I'm not very sure if grunt is the right word. I think a grunt is louder and comes mainly from the throat. I'm looking for a word that expresses a softer noise.
Delightful question. A nasal pragmatic (subset backchannelling!) marker! Is this the ah-hah? (rising intonation, if that's the right word for hums). Or the mono'syllabic' Hmm.
Other 'hums' include
the equivalent of 'What!?' (hmMMM?).
The 'No!' (uh-huh; falling intonation).
The eureka Aah.
The discovery of a sinister plot by the kids (A-hah)'
The disgusted eugh.
The 'haven't a clue' (single high note) (possibly rendered Huh?.)
If bib wants a nasal dictionary, he can write his own.
The act of sniffing might suggest what you are describing:
- To inhale a short, audible breath through the nose, as in smelling something.
I don't think there's a word for it - you'd just imitate the noise.
(I'm assuming here...)
The sound you're referring to is usually spelled "Mm" or "Mm-hmm" (the latter to differentiate it from the sound you make when something smells yummy)
"I don't know if he was listening, or not - he just 'Mm-hmm'd' at me - so, I'll take it as a 'yes'." (US)