muttering vs murmur

muttering and murmur both are nouns.

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

As nouns, muttering and murmur are synonyms defined as:

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

As nouns, muttering and murmur are synonyms defined as:

muttering and murmur: a low continuous indistinct sound; often accompanied by movement of the lips without the production of articulate speech

Other synonyms of muttering include murmuration, murmuring, mussitation, mutter.

muttering (noun)	murmur (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 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

muttering (verb)	murmur (verb)
	speak softly or indistinctly
	make complaining remarks or noises under one's breath