sing vs hum

sing is not a noun while hum is a noun.

Word	Noun	Adjective	Verb	Adverb
sing	No	No	Yes	No
hum	Yes	No	Yes	No

As verbs, hum is a hyponym of sing; that is, hum is a word with a more specific, narrower meaning than sing:

sing (noun)	hum (noun)
	a humming noise
	the state of being or appearing to be actively engaged in an activity

sing (verb)	hum (verb)
make a whining, ringing, or whistling sound	sing with closed lips
divulge confidential information or secrets	make a low continuous sound
to make melodious sounds	sound with a monotonous hum
produce tones with the voice	be noisy with activity
deliver by singing