murmuring vs grumbling

murmuring and grumbling both are nouns.

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

As nouns, murmuring and grumbling are synonyms defined as:

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

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

murmuring (noun)	grumbling (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 loud low dull continuous noise

murmuring (adjective)	grumbling (adjective)
making a low continuous indistinct sound	continuous full and low-pitched throbbing sound