@tonezandi Thanks for reaching out!
This is a really interesting one! Your best bet would be to take it in to your local bike shop and have a tech take it for a ride and see if they can pinpoint the source of the sound. Are you noticing anything else that is different about the bike when you're pedaling in those gears? Does it occur at a particular part of the pedal stroke? Has it always made that sound or did it just start?