hiss vs let loose

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

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

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

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

hiss (noun)	let loose (noun)
a cry or noise made to express displeasure or contempt
a fricative sound (especially as an expression of disapproval)

hiss (verb)	let loose (verb)
make a sharp hissing sound, as if to show disapproval	express audibly; utter sounds (not necessarily words)
express or utter with a hiss	turn loose or free from restraint
show displeasure, as after a performance or speech
move with a whooshing sound