Just put a SRAM PG970 11-32 cassette PC971 chain and a Shimano STX derailleur on my bike. All three are new from the box. Shifters are STI and a year old. All shifts up and down are smooth an there is no unusual noise in any gear except the middle (5th) gear which happens to be my favorite gear. There is a tick which is in time with my cadence and and the level of noise increases with peddle effort. When my peddling decreases the ticking stops. Even the best tuning of the rear dérailleur won't remove the noise. My LBS mechanic (who I have complete confidence) tells me the ticking will disappear with use. I guess I can live with the noise or work around it by using my large chain ring but I'm wondering if I just have a finicky component.
The tick happens with each stroke so it's got nothing to do with the chain or the rear cogs.
Since it goes away on the center cog, it must be related to the diagonal chanin line.
The chain has only one object with which it has contact, your front deraileur cage.
Since pedal exertion changes the noise, you must be altering the chainring's parallelism. You're either deforming the rings or the bottom bracket is twisting in the frame.
Without seeing your bike, the above are the worst kind of idle speculation but they represent a logical approach to finding a mechanical problem by eliminating that which simply cannot be. When you eliminate the obvious and the logical, of course, the only thing left is the impossible or the magical.
