sputter vs let loose

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

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

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

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

sputter (noun)	let loose (noun)
an utterance (of words) with spitting sounds (as in rage)
the noise of something spattering or sputtering explosively

sputter (verb)	let loose (verb)
spit up in an explosive manner	express audibly; utter sounds (not necessarily words)
cause to undergo a process in which atoms are removed	turn loose or free from restraint
utter with a spitting sound, as if in a rage
climb awkwardly, as if by scrambling
make an explosive sound