bullwhipping vs knocking

bullwhipping

noun
  • A beating with a bullwhip. 

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 bullwhipping and knocking occurred in a corpus of books? (source: Google Ngram Viewer )