Answer: mutter

AnswerCrossword Clue
MUTTERGrumble.
mutterto murmur (to make a subdued muttered sound)
mutterSpeak privately or unofficially about someone or something; spread rumors
mutterSay something in a low or barely audible voice, esp. in dissatisfaction or irritation
MUTTERExpress irritation, maybe
MUTTERSay inaudibly
MUTTER[Grumble, grumble, grumble]
MUTTERNot say clearly
MUTTERSpeak low.
MUTTERSpeak indistinctly.
MUTTERBe hard to hear
MUTTERGrumble quietly
MUTTERMurmur indistinctly
MUTTERSpeak indistinctly
MUTTERSay under one's breath
MUTTERComplain under one's breath
MUTTERComplain in a murmur
MUTTERUnder-one's-breath sound
MUTTERGrumble