grumble vs murmur

grumble and murmur both are nouns.

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

As nouns, grumble and murmur are synonyms defined as:

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

As verbs, grumble and murmur are synonyms defined as:

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

Other synonyms of grumble include croak, gnarl, mutter.

grumble (noun)	murmur (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 schwa that is incidental to the pronunciation of a consonant
	a low continuous indistinct sound; often accompanied by movement of the lips without the production of articulate speech
	an abnormal sound of the heart; sometimes a sign of abnormal function of the heart valves

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