churr vs whirr

churr

verb
  • To make the low vocal sound of some birds. 

noun
  • A low vocal sound made by some birds. 

whirr

verb
  • To make a sibilant buzzing or droning sound. 

  • To cause (something) to make such a sound. 

  • To move or vibrate (something) with a buzzing sound. 

noun
  • A sibilant buzz or vibration; the sound of something in rapid motion. 

  • A bustle of noise and excitement. 

How often have the words churr and whirr occurred in a corpus of books? (source: Google Ngram Viewer )