grumbling vs muttering

grumbling and muttering both are nouns.

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

As nouns, grumbling and muttering are synonyms defined as:

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

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

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