mutter vs grumbling

mutter and grumbling both are nouns.

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

As nouns, mutter and grumbling are synonyms defined as:

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

mutter (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

mutter (adjective)	grumbling (adjective)
	continuous full and low-pitched throbbing sound

mutter (verb)	grumbling (verb)
make complaining remarks or noises under one's breath
talk indistinctly; usually in a low voice