muttering vs murmuring

muttering and murmuring both are nouns.

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

As nouns, muttering and murmuring are synonyms defined as:

muttering and murmuring: a complaint uttered in a low and indistinct tone

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

As nouns, muttering and murmuring are synonyms defined as:

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

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

muttering (noun)	murmuring (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 low continuous indistinct sound; often accompanied by movement of the lips without the production of articulate speech

muttering (adjective)	murmuring (adjective)
	making a low continuous indistinct sound