Our 2008 Cannondale Road Tandem 3 has been making some unusual drivetrain noise recently. Whenever I shift into the second largest rear cog (30t, cassette is a 9spd 11-34), regardless of which chainring I'm in (53/39/30) I get a grinding noise from the rear portion of the bike--either the RD (Shimano Deore LX), cassette or hub (Shimano Tandem 40h).

No other gear combinations make this sound. To the best of my knowledge, I have adjusted the derailleurs and limit screws correctly. This is evidenced (IMO) by the crisp, quiet shifting in every other gear combination. There seems to be no debris either in the RD or near the 30t cog, and the chain is clean and relatively new.

Lastly, the grinding sound only happens under load--i.e. when I put the bike in a stand, shift into the 53/30, 39/30 or 30/30, and use my hand to turn the cranks, I can't reproduce the sound. However, when pedaling under load, the grinding sound happens whenever I shift into the 30t, regardless of whether I'm shifting from an easier gear or a harder gear.

Any advice would be greatly appreciated. Thanks!