During rehearsals for the SATB choir I'm part of, we are sometimes asked to hum the melody of a song, or even scales as a warm-up exercise.
The humming-technique is sometimes explained as "keeping the mouth fully closed, but relaxed", producing what I would call the 'humming sound': 'mmmm'. This is confirmed by the remark: "when your lips vibrate, you're doing it right".
I notice that many people in my group and even the conductor are actually 'singing', producing an 'oo' sound (like in 'zoo') or 'oo' (like in 'none')*. This is done with a slightly open mouth.
My assumption is that it's hard to make herself heard over a group of 40+ in a large hall when ourconductor is explaining the exercise using strict humming. For the other singers I'm not sure why they would do that.
This led me to the question: "What sound should humming make?".
*I don't know the phonetic signs to indicate this, feel free to edit!