muttering vs grumble

muttering and grumble both are nouns.

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

As nouns, muttering and grumble are synonyms defined as:

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

muttering (noun)	grumble (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 (verb)	grumble (verb)
	make complaining remarks or noises under one's breath
	show one's unhappiness or critical attitude
	to utter or emit low dull rumbling sounds
	make a low noise