"grumbling" in the noun sense
1. rumble, rumbling, grumble, grumbling
a loud low dull continuous noise
"they heard the rumbling of thunder"
2. grumble, grumbling, murmur, murmuring, mutter, muttering
a complaint uttered in a low and indistinct tone
"grumbling" in the verb sense
1. grouch, grumble, scold
show one's unhappiness or critical attitude
"He scolded about anything that he thought was wrong"
"We grumbled about the increased work load"
2. murmur, mutter, grumble, croak, gnarl
make complaining remarks or noises under one's breath
"she grumbles when she feels overworked"
3. grumble, growl, rumble
to utter or emit low dull rumbling sounds
"he grumbled a rude response"
"Stones grumbled down the cliff"
4. rumble, grumble
make a low noise
"grumbling" in the adjective sense
1. grumbling, rumbling
continuous full and low-pitched throbbing sound
"the rumbling rolling sound of thunder"
Source: WordNet® (An amazing lexical database of English)
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