humming vs singing

humming and singing both are nouns.

Word	Noun	Adjective	Verb	Adverb
humming	Yes	No	No	No
singing	Yes	Yes	No	No

As nouns, singing is a hypernym of humming; that is, singing is a word with a broader meaning than humming:

Other hypernyms of humming include vocalizing.

humming (noun)	singing (noun)
the act of singing with closed lips	the act of singing vocal music
a humming noise	disclosing information or giving evidence about another

humming (adjective)	singing (adjective)
	smooth and flowing