hum in WordNet English dictionary

1. the state of being or appearing to be actively engaged in an activity
   "they manifested all the busyness of a pack of beavers"; "there is a constant hum of military preparation"
   busyness  hum 
   
2. a humming noise
   "the hum of distant traffic"
   hum  humming 
   

1. be noisy with activity
   "This office is buzzing with activity"
   hum  buzz  seethe 
   
2. sing with closed lips
   "She hummed a melody"
   hum 
   
3. make a low continuous sound
   "The refrigerator is humming"
   hum 
   
4. sound with a monotonous hum
   hum  thrum