[tomcho]

My first thought was the front derailleur but you are saying the noise only happens on the middle gears of the rear cog? It's silent on the smallest and largest? If so it's not your FD.

What do you mean by middle gears?

 

[tomcho]

I'm far from an wrenching expert, but that seems like a very odd problem. Typically noises from rubbing either against the front derailleur or other rings in the rear cog happen at extremes; i.e the smallest or largest rear gears, due to the chain angle. To only have noise in the middle gears is a head scratcher for me.

I'm sure someone who knows a lot more then myself will chime in.