coo vs let out

coo is a noun but let out is not a noun.

Word	Noun	Adjective	Verb	Adverb
coo	Yes	No	Yes	No
let out	No	No	Yes	No

As verbs, let out is a hypernym of coo; that is, let out is a word with a broader meaning than coo:

Other hypernyms of coo include emit, let loose, utter.

coo (noun)	let out (noun)
the sound made by a pigeon

coo (verb)	let out (verb)
cry softly, as of pigeons	make (clothes) larger
speak softly or lovingly	make known to the public information that was previously known only to a few people or that was meant to be kept a secret
	express audibly; utter sounds (not necessarily words)
	bring out of a specific state