grumble vs let out

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

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

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

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

grumble (noun)	let out (noun)
a complaint uttered in a low and indistinct tone
a loud low dull continuous noise

grumble (verb)	let out (verb)
make complaining remarks or noises under one's breath	make (clothes) larger
show one's unhappiness or critical attitude	make known to the public information that was previously known only to a few people or that was meant to be kept a secret
to utter or emit low dull rumbling sounds	express audibly; utter sounds (not necessarily words)
make a low noise	bring out of a specific state