complaint vs grumbling

complaint and grumbling both are nouns.

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

As nouns, grumbling is a hyponym of complaint; that is, grumbling is a word with a more specific, narrower meaning than complaint:

Other hyponyms of complaint include grievance, grumble, murmur, murmuring, mutter, muttering, jeremiad, kvetch, pet peeve, whimper, whine.

complaint (noun)	grumbling (noun)
an expression of grievance or resentment	a complaint uttered in a low and indistinct tone
(formerly) a loud cry (or repeated cries) of pain or rage or sorrow	a loud low dull continuous noise
(civil law) the first pleading of the plaintiff setting out the facts on which the claim for relief is based
(criminal law) a pleading describing some wrong or offense
an often persistent bodily disorder or disease; a cause for complaining

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