pelt vs rain down

pelt is a noun but rain down is not a noun.

Word	Noun	Adjective	Verb	Adverb
pelt	Yes	No	Yes	No
rain down	No	No	Yes	No

As verbs, rain down is a hypernym of pelt; that is, rain down is a word with a broader meaning than pelt:

Other hypernyms of pelt include rain.

pelt (noun)	rain down (noun)
body covering of a living animal
the dressed hairy coat of a mammal

pelt (verb)	rain down (verb)
attack and bombard with or as if with missiles	precipitate as rain
cast, hurl, or throw repeatedly with some missile
rain heavily