The term 'monotonous sound' can be described by three five-letter crossword solutions: DRONE, THRUM, and CHANT. A DRONE is a continuous, low humming sound, often produced by an engine or other machinery. A THRUM is a low, vibrating sound, usually created by a stringed instrument. Lastly, a CHANT is a repetitive vocalization, often used in religious ceremonies or rituals. All three of these sounds are typically considered to be monotonous and lack variation in pitch or tone.