hmm vs hrm

hmm

intj
  • Indicating thinking or pondering. 

  • A demand for an answer to a question. 

verb
  • To make a thoughtful humming noise. 

hrm

intj
  • Expresses a moment of particular curiosity or thoughtfulness. 

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