monotonous

Adjective

• sounded or spoken in a tone unvarying in pitch
  • the owl's faint monotonous hooting
    

• tediously repetitious or lacking in variety; all work and no play"
  • a humdrum existence
    
  • nothing is so monotonous as the sea
    

Related words

• Synonyms: 
  flat , monotone , monotonic , humdrum