mutter vs grumble

mutter and grumble both are nouns.

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

As nouns, mutter and grumble are synonyms defined as:

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

As verbs, mutter and grumble are synonyms defined as:

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

Other synonyms of mutter include croak, gnarl, murmur.

mutter (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

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