eat away at vs gnaw away at

eat away at

verb
  • To erode. 

  • To consume or use up, erode or wear away gradually. 

  • To cause to feel uneasy, troubled, worried, guilty, or vexed; to bother about something. 

gnaw away at

How often have the words eat away at and gnaw away at occurred in a corpus of books? (source: Google Ngram Viewer )