palpitation vs pitter-patter

palpitation

noun
  • An abnormal beating of the heart that may be perceived by the patient, a result of excitement, exertion, or illness. 

pitter-patter

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

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