hum
v. B1 Intermediate US //ˈhəm// UK //hˈʌm// hum
v. to make a low, continuous sound with your lips closed. You might do this when you are thinking or when you know the tune of a song but not the words.
v. to emit a low, steady, continuous sound, typically produced by vibrating the vocal cords with the mouth closed. Often describes the sound of machinery or a busy environment.
She likes to hum a happy tune while she works.
The old refrigerator started to hum loudly in the middle of the night.
The entire server room began to hum with a low-frequency vibration that seemed to resonate through the floorboards of the office above.
Usage
Intransitive when describing the sound itself; transitive when the object is a specific melody ('hum a song').