echo vs let loose

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

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

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

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

echo (noun)	let loose (noun)
an imitation or repetition
the repetition of a sound resulting from reflection of the sound waves
a reflected television or radio or radar beam
a close parallel of a feeling, idea, style, etc.
a reply that repeats what has just been said

echo (verb)	let loose (verb)
to say again or imitate	express audibly; utter sounds (not necessarily words)
ring or echo with sound	turn loose or free from restraint
bring to mind