chirring vs chirrup

chirring

noun
  • The sound of a chirr. 

chirrup

noun
  • A series of chirps, clicks or clucks. 

  • A brief, high-pitched, insignificant statement. 

verb
  • To express by chirping. 

  • To make a series of chirps, clicks, or clucks. 

  • To quicken or animate by chirping. 

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