monotonous

adjective2

1. •
   Having an unvarying pitch or tone.

   • "But my daughter—but the Lady Eveline"—said the Fleming, with some slight change in his monotonous voice, which seemed to express doubt and perplexity—"You are cruel conquerors, and—" — 1825
2. •
   Tedious, repetitious, or lacking in variety.

   • [L]istening for a moment, she heard the monotonous and heavy tread of the soldier echoing through the passage. — 1834