pipe up vs holler

pipe up is not a noun while holler is a noun.

Word	Noun	Adjective	Verb	Adverb
pipe up	No	No	Yes	No
holler	Yes	No	Yes	No

As verbs, holler is a hypernym of pipe up; that is, holler is a word with a broader meaning than pipe up:

Other hypernyms of pipe up include call, cry, hollo, scream, shout, shout out, squall, yell.

pipe up (noun)	holler (noun)
	a very loud utterance (like the sound of an animal)
	a small valley between mountains