monotonous vs monotone

monotonous is not a noun while monotone is a noun.

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

As adjectives, monotonous and monotone are synonyms defined as:

Other synonyms of monotonous include flat, monotonic.

monotonous (noun)	monotone (noun)
	an unchanging intonation
	a single tone repeated with different words or different rhythms (especially in rendering liturgical texts)

monotonous (adjective)	monotone (adjective)
tediously repetitious or lacking in variety	of a sequence or function; consistently increasing and never decreasing or consistently decreasing and never increasing in value
sounded or spoken in a tone unvarying in pitch	sounded or spoken in a tone unvarying in pitch