pitter-patter vs thud

pitter-patter

noun
  • A soft, percussive sound, as of tiny feet, or of rain on a rooftop. 

thud

noun
  • The sound of a dull impact. 

  • A slower, dull impact with a wide surface area. 

  • A hard, dull impact. 

verb
  • To make the sound of a dull impact. 

How often have the words pitter-patter and thud occurred in a corpus of books? (source: Google Ngram Viewer )