Possible Clues:

• Grumble has come from middle of stomach, say
• Grouse quietly
• Grumble in secret
• Male to speak — inaudibly so?
• Talk under one's breath
• Grumble under one's breath
• Speak indistinctly in mass, say
• Murmur complaints
• Mouth's opening and nothing more, say, for this?
• Speak unclearly