grumbling vs mutter

grumbling and mutter both are nouns.

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

As nouns, grumbling and mutter are synonyms defined as:

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

grumbling (noun)	mutter (noun)
a complaint uttered in a low and indistinct tone	a complaint uttered in a low and indistinct tone
a loud low dull continuous noise	a low continuous indistinct sound; often accompanied by movement of the lips without the production of articulate speech

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

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