roar vs let loose

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

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

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

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

roar (noun)	let loose (noun)
a very loud utterance (like the sound of an animal)
a deep prolonged loud noise
the sound made by a lion

roar (verb)	let loose (verb)
laugh unrestrainedly and heartily	express audibly; utter sounds (not necessarily words)
make a loud noise, as of wind, water, or vehicles	turn loose or free from restraint
emit long loud cries
utter words loudly and forcefully
make a loud noise, as of animal
act or proceed in a riotous, turbulent, or disorderly way