beep vs knocking

beep

noun
  • The sound produced by the horn of a car, or any similar sound. 

  • A message sent to a pager device. 

  • A short, electronically produced tone. 

verb
  • To sound (something that makes a beep). 

  • To contact (someone via) a pager device. 

  • To telephone a person, only allowing the phone to ring once, in order to request a call back. 

  • To produce a beep. 

  • To have sexual intercourse with - referring to the bleep tone used to censor obscene words in broadcasts 

knocking

noun
  • An act in which something is knocked on, or the sound thus produced 

  • A repetitive ping, knock, or similar sound coming from an engine in which there are repeated uncontrolled explosions in the combustion chamber. 

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