grumble vs mutter

grumble and mutter both are nouns.

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

As nouns, grumble and mutter are synonyms defined as:

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

As verbs, grumble and mutter are synonyms defined as:

grumble and mutter: make complaining remarks or noises under one's breath

Other synonyms of grumble include croak, gnarl, murmur.

grumble (noun)	mutter (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)	mutter (verb)
make complaining remarks or noises under one's breath	make complaining remarks or noises under one's breath
show one's unhappiness or critical attitude	talk indistinctly; usually in a low voice
to utter or emit low dull rumbling sounds
make a low noise