grumble vs muttering

grumble and muttering both are nouns.

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

As nouns, grumble and muttering are synonyms defined as:

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

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

grumble (verb)	muttering (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