muttering vs grumbling

muttering and grumbling both are nouns.

Word	Noun	Adjective	Verb	Adverb
muttering	Yes	No	No	No
grumbling	Yes	Yes	No	No

As nouns, muttering and grumbling are synonyms defined as:

muttering and grumbling: a complaint uttered in a low and indistinct tone

Other synonyms of muttering include grumble, murmur, murmuring, mutter.

muttering (noun)	grumbling (noun)
a complaint uttered in a low and indistinct tone	a complaint uttered in a low and indistinct tone
a low continuous indistinct sound; often accompanied by movement of the lips without the production of articulate speech	a loud low dull continuous noise

muttering (adjective)	grumbling (adjective)
	continuous full and low-pitched throbbing sound