Start of a response to a brainteaser

• "I wonder ..."
• Thoughtful sound
• Thinker's sound
• 'Very interesting'
• Exclamation of contemplation
• "Let me think about that …"
• What Rodin's Thinker is thinking?
• "Let me think about that"
• "I dunno ..."

Douglas Harper's Etymology Dictionary Word definitions in Douglas Harper's Etymology Dictionary
representative of a sound made during contemplation or showing mild disapproval, attested from 1868, but this is probably a variation of the hum attested in similar senses from 1590s.