hum vs purr

hum

verb
  • To produce low sounds which blend continuously 

  • To express by humming. 

  • To reek, smell bad. 

  • To make a sound from the vocal chords without pronouncing any real words, with one's lips closed. 

  • To drone like certain insects naturally do in motion, or sounding similarly 

  • To buzz, be busily active like a beehive 

noun
  • Unpleasant odour. 

  • A phenomenon, or collection of phenomena, involving widespread reports of a persistent and invasive low-frequency humming, rumbling, or droning noise not audible to all people. 

  • Busy activity, like the buzz of a beehive. 

  • A hummed tune, i.e. created orally with lips closed. 

  • An often indistinct sound resembling human humming. 

intj
  • Synonym of um: a noise indicating doubt, uncertainty, &c. 

  • Synonym of hmm: a noise indicating thought, consideration, &c. 

purr

verb
  • To make a low and consistent rumbling sound. 

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

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

  • 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 

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