hum vs knoll

hum

verb
  • To express by humming. 

  • To produce low sounds which blend continuously 

  • 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 

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

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

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. 

knoll

verb
  • To sound (something) like a bell; to knell. 

  • To ring (a bell) mournfully; to knell. 

  • To call (someone, to church) by sounding or making a knell (as a bell, a trumpet, etc). 

  • To arrange related objects in parallel or at 90 degree angles. 

noun
  • A rounded, underwater hill with a prominence of less than 1,000 metres, which does not breach the water's surface. 

  • A small mound or rounded hill. 

  • A knell. 

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