grumbling
grum-bling
noun
A low, continuous sound, or the act of complaining quietly and persistently.
"There was much grumbling about the new rules."
verb
Making a low, continuous sound, or complaining quietly and persistently about something.
"He was grumbling about the cold weather."
Thesaurus
Synonyms: rumble, rumbling, grumble, full, murmuring, murmur, mutter, muttering