murmur vs grumble

murmur and grumble both are nouns.

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

As nouns, murmur and grumble are synonyms defined as:

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

As verbs, murmur and grumble are synonyms defined as:

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

Other synonyms of murmur include croak, gnarl, mutter.

murmur (noun)	grumble (noun)
a complaint uttered in a low and indistinct tone	a complaint uttered in a low and indistinct tone
a schwa that is incidental to the pronunciation of a consonant	a loud low dull continuous noise
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

murmur (verb)	grumble (verb)
speak softly or indistinctly	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
	to utter or emit low dull rumbling sounds
	make a low noise