The noun has 2 meanings:
Meaning #1:
a loud low dull continuous noise
Synonyms: rumble, rumbling, grumble
Meaning #2:
a complaint uttered in a low and indistinct tone
Synonyms: grumble, murmur, murmuring, mutter, muttering
The adjective grumbling has 2 meanings:
Meaning #1:
continuous full and low-pitched throbbing sound
Synonym: rumbling
Meaning #2:
muttering discontentedly
