Answer:

muttered

AnswerCrossword Clue
MUTTEREDComplained under one's breath
MUTTEREDSounded unhappy
MUTTEREDTalked indistinctly.
MUTTEREDGrumbled.
MUTTEREDGrumbled indistinctly
MUTTEREDGrumbled