rain down vs rain

rain down is not a noun while rain is a noun.

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

As verbs, rain down and rain are synonyms defined as:

rain down (noun)	rain (noun)
	anything happening rapidly or in quick successive
	water falling in drops from vapor condensed in the atmosphere
	drops of fresh water that fall as precipitation from clouds

rain down (verb)	rain (verb)
precipitate as rain	precipitate as rain