rumble vs let out

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

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

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

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

rumble (noun)	let out (noun)
a fight between rival gangs of adolescents
a servant's seat (or luggage compartment) in the rear of a carriage
a loud low dull continuous noise

rumble (verb)	let out (verb)
to utter or emit low dull rumbling sounds	make (clothes) larger
make a low noise	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