I used a triple crank for several years. If you consider the chainline in the middle ring and largest cog, you realize that is is nearly as extreme as the big ring and largest cog on a double. That is considered to be an off limits combination. I tried to never used the middle ring and largest cog. It's also why I chose a 39T middle ring, so the 39/23 was as low as a 42/25.
Since the problem only occurs under torque, it's even more likely that it's chainline issue. Surely you can tell the difference between the chain and cogs making noise versus something like the RD cage hitting the spokes.
Last edited by DaveSSS; 11-20-09 at 09:53 AM.