rumble vs let loose

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

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

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

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

rumble (noun)	let loose (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 loose (verb)
to utter or emit low dull rumbling sounds	express audibly; utter sounds (not necessarily words)
make a low noise	turn loose or free from restraint