"I need to think about that ..."

• 'Let me think, ....'
• Chin scratcher's utterance
• "Things that Make You Go ___"
• “That’s an interesting thought ...”
• "Good question ..."
• Ponderer's sound
• Muser's sound
• "Let's see ... "
• "Thinking ..."

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.