quiet down vs hush

quiet down is not a noun while hush is a noun.

Word	Noun	Adjective	Verb	Adverb
quiet down	No	No	Yes	No
hush	Yes	No	Yes	No

As verbs, quiet down and hush are synonyms defined as:

Other synonyms of quiet down include pipe down, quiesce, quiet, quieten.

quiet down (noun)	hush (noun)
	(poetic) tranquil silence

quiet down (verb)	hush (verb)
become quiet or quieter	become quiet or still; fall silent
	cause to be quiet or not talk
	run water over the ground to erode (soil), revealing the underlying strata and valuable minerals
	wash by removing particles
	become quiet or quieter