That's a tough one. Does it squeak with front shifts or rear shifts? Between all gears or just some? Can you hear it with the bike stationary (on a repair stand), or only when riding and under pressure? Try shifting with the rear wheel removed and see if it still squeaks. You could try greasing the cables (remove them from the casing, grease, and reinsert). Maybe a drop of oil in the right place in the levers (probably where the lever pivots). Sorry I can't be more helpful.