er vs hrm

er

intj
  • Said when hesitating in speech. 

noun
  • The name of the Cyrillic script letter Р / р. 

verb
  • To utter the word "er" when hesitating in speech, found in the phrase um and er. 

hrm

intj
  • Expresses a moment of particular curiosity or thoughtfulness. 

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