humdrum vs monotonous

humdrum is a noun but monotonous is not a noun.

Word	Noun	Adjective	Verb	Adverb
humdrum	Yes	Yes	No	No
monotonous	No	Yes	No	No

As adjectives, humdrum and monotonous are synonyms defined as:

humdrum (noun)	monotonous (noun)
the quality of wearisome constancy, routine, and lack of variety

humdrum (adjective)	monotonous (adjective)
tediously repetitious or lacking in variety	tediously repetitious or lacking in variety
not challenging; dull and lacking excitement	sounded or spoken in a tone unvarying in pitch