murmur vs muttering

murmur and muttering both are nouns.

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

As nouns, murmur and muttering are synonyms defined as:

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

As nouns, murmur and muttering are synonyms defined as:

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

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

murmur (noun)	muttering (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 low continuous indistinct sound; often accompanied by movement of the lips without the production of articulate speech
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)	muttering (verb)
speak softly or indistinctly
make complaining remarks or noises under one's breath