rain down vs spit

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

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

As verbs, spit is a hyponym of rain down; that is, spit is a word with a more specific, narrower meaning than rain down:

rain down (noun)	spit (noun)
	the act of spitting (forcefully expelling saliva)
	a skewer for holding meat over a fire
	a clear liquid secreted into the mouth by the salivary glands and mucous glands of the mouth; moistens the mouth and starts the digestion of starches
	a narrow strip of land that juts out into the sea

rain down (verb)	spit (verb)
precipitate as rain	expel or eject (saliva or phlegm or sputum) from the mouth
	utter with anger or contempt
	drive a skewer through
	rain gently