howl vs yammer

howl is a noun but yammer is not a noun.

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

As verbs, howl and yammer are synonyms defined as:

Other synonyms of howl include wrawl, yowl.

howl (noun)	yammer (noun)
a long loud emotional utterance
a loud sustained noise resembling the cry of a hound
the long plaintive cry of a hound or a wolf

howl (verb)	yammer (verb)
laugh unrestrainedly and heartily	complain whiningly
make a loud noise, as of wind, water, or vehicles	cry loudly, as of animals
emit long loud cries
cry loudly, as of animals