pit-a-pat vs pitter-patter

pit-a-pat

noun
  • A series of quick tapping sounds 

adv
  • With a quick tapping sound 

verb
  • To make a series of quick tapping sounds; to palpitate 

pitter-patter

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

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