monotonous vs humdrum

monotonous is not a noun while humdrum is a noun.

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

As adjectives, monotonous and humdrum are synonyms defined as:

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

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