Answer: muttering

AnswerCrossword Clue
MUTTERINGGrumbling under one's breath
MUTTERINGSpeaking indistinctly.
MUTTERINGCertain complaining
mutteringSay something in a low or barely audible voice, esp. in dissatisfaction or irritation
mutteringSpeak privately or unofficially about someone or something; spread rumors
mutteringMUTTER, to murmur (to make a subdued muttered sound)