grumble vs let loose

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

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

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

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

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

grumble (verb)	let loose (verb)
make complaining remarks or noises under one's breath	express audibly; utter sounds (not necessarily words)
show one's unhappiness or critical attitude	turn loose or free from restraint
to utter or emit low dull rumbling sounds
make a low noise