"I dunno ..."

• 'Let me think, ....'
• Ponderer's sound
• "Let's see ..."
• "Let me think ... "
• What a deep thinker might mumble
• “That’s an interesting thought ...”
• Exclamation of contemplation
• ''Very interesting . . .''
• "Let's see ... "

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.