purring vs whirring

purring

noun

  • A sound that purrs. 

  • Synonym of shin-kicking 

whirring

noun

  • A whirr, the sounds of a whirr. 

adj

  • Producing a whirr; giving off a whirring noise. 

  • Spinning rapidly. 

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