Is 'hmmm?' an interjection? If not, what is it?

I hope this isn't obvious (although maybe it's better if it is)! I'm working with an interview transcript and I have a participant who says 'Hmmm?' in a way that I want to discuss. Unfortunately, I don't know how to refer to it. Here's an example of what he's saying (note, he's a non-native speaker):

• Interviewer: Did you help them yourself?
• Participant: Hmmmm?
• I: Did you personally help them, or was it..
• P: Oh yes, I...

And so on. I don't think it's a vocalized pause or a filler word, since he's not speaking at length, just prompting the interviewer to provide clarification. Does this type of sound/question have a name, or am I completely off track?

Solution 1:

I would consider it to be an interjection. — M-W

a word or cry (as “ouch”) expressing sudden or strong feeling

"a chorus of angry interjections greeted the announcement that our flight would be delayed"

Word: mmm, hmm, mhm, uhhu.. etc.
Translation: Yes
Example: "Do you think so too?" "Mhm"
Meaning: Agreement, acknowledgement

See more interjections in this link: "Dictionary of interjections"