What is the meaning of humming?

noun

• A humming noise.
  • usage: "the hum of distant traffic"
  • synonyms: hum
• The act of singing with closed lips.

verb

• Sing with closed lips.
  • usage: "She hummed a melody"
  • synonyms: hum
• Be noisy with activity.
  • usage: "This office is buzzing with activity"
  • synonyms: hum, buzz, seethe
• Sound with a monotonous hum.
  • synonyms: hum, thrum
• Make a low continuous sound.
  • usage: "The refrigerator is humming"
  • synonyms: hum