humming vs tremble

humming

noun

  • The sound of something that hums; a hum. 

adj

  • Making a hum sound. 

tremble

noun

  • A shake, quiver, or vibration. 

verb

  • To shake, quiver, or vibrate. 

  • To fear; to be afraid. 

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