mutter vs secret message

mutter

noun
  • A repressed or obscure utterance; an instance of muttering. 

  • Peas. 

verb
  • To speak softly and incoherently, or with imperfect articulations. 

  • To make a sound with a low, rumbling noise. 

  • To utter words, especially complaints or angry expressions, indistinctly or with a low voice and lips partly closed; to say under one's breath. 

secret message

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