Synonyms for Grumbling (same or very similar meaning)
WordNet sense 1 (a loud low dull continuous noise):
grumble, rumble, rumbling
WordNet sense 2 (a complaint uttered in a low and indistinct tone):
murmur, murmuring, mutter, muttering
WordNet sense 3 ((of sound) having marked depth and body):
full
WordNet sense 4 (expressing pain or dissatisfaction of resentment):
complaining, complaintive
From the ODE community, based on WordNet
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