bellowing vs holler

bellowing and holler both are nouns.

Word	Noun	Adjective	Verb	Adverb
bellowing	Yes	No	No	No
holler	Yes	No	Yes	No

As nouns, bellowing and holler are synonyms defined as:

bellowing and holler: a very loud utterance (like the sound of an animal)

Other synonyms of bellowing include bellow, holla, hollering, hollo, holloa, roar, roaring, yowl.

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