howl vs yowl

howl and yowl both are nouns.

Word	Noun	Adjective	Verb	Adverb
howl	Yes	No	Yes	No
yowl	Yes	No	Yes	No

As verbs, howl and yowl are synonyms defined as:

Other synonyms of howl include wrawl, yammer.

howl (noun)	yowl (noun)
a long loud emotional utterance	a very loud utterance (like the sound of an animal)
a loud sustained noise resembling the cry of a hound
the long plaintive cry of a hound or a wolf

howl (verb)	yowl (verb)
laugh unrestrainedly and heartily	utter shrieks, as of cats
make a loud noise, as of wind, water, or vehicles	cry loudly, as of animals
emit long loud cries
cry loudly, as of animals