purr vs sing low

purr

verb
  • Of a cat, to make a vibrating sound in its throat when contented. 

  • To make a vibrating throaty sound, as from pleasure. 

  • To make a low and consistent rumbling sound. 

  • To say (something) in a throaty, seductive manner. 

intj
  • Throat vibrating sound made by a cat. 

noun
  • The vibrating sound made by a cat in its throat when contented. 

  • A throaty, seductive sound of pleasure made by a person. 

  • The low consistent rumble made by an engine at slow speed 

sing low

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