What is the meaning of whirr?

noun

• Sound of something in rapid motion.
  • usage: "whir of a bird's wings"; "the whir of the propellers"
  • synonyms: whir, whirring, birr

verb

• Make a soft swishing sound.
  • usage: "the motor whirred"; "the car engine purred"
  • synonyms: whizz, whiz, whir, birr, purr
• Make a vibrant sound, as of some birds.
  • synonyms: churr