noun / verb
hum
HUM/hʌm/
To emit a quiet continuous vibrating sound.
He hums a quiet tune, keeping time with one hand.

Etymology
Middle English hommen described murmuring to cover embarrassment, and the verb later meant buzzing or droning. Probably imitative in origin, it came to include singing with closed lips.
In context
Gently close the lips and hum on an Mmmm Have some space between your back molars Feel the resonance or buzzing at the lips
More examples
- The refrigerator hums softly in the corner.
- A steady hum comes from the air conditioner.
- The workshop hums with activity all morning.
Forms
hum · hums · hummed · humming