Possible Clues:

• Grumbles
• Grumbles quietly
• Grumps
• Grumbles sotto voce
• Say 'Put in writing,' evoking grumbles
• Says 'You no-good son of a ...,' say
• 'Trust me!' comic says none too clearly
• How grand when one speaks in such a low way!
• This loses its head and speaks in a low sort of way
• Complains sotto voce