Originally Posted by hhliao

I tried 3 things tonight.

I tuned the tension such that upshifting was precise, and, as described in my original post, downshifting tended to over-shift and made noise. I then gently pushed on the thumb lever, and, indeed, the noise went away. So I took FBinNY's advice, and took the shifter apart, cleaned the outside of the g-spring carrier and the inside of the lever housing. Put everything together, the shifting problem persists...

I also tried cutting a longer loop around rear derailleur, and the problem still persisted. As a desperate move, I started squirting my favorite chain lube (Boeshield T-9) in the housing. The shifting behaved the same way.

So frustrating to have a long anticipated project to go like this...