Nothing left but to adjust or replace the belt. We could, perhaps, work out a perfect argument and that still would not change what the machine does. Decide what you want to do now. Tha ball is in your court. Good luck.UPDATE :
Ok, so I tried what you said. I sprayed a good amount of belt dressing on the belt. Nothing has changed. Still can hear the noise sometimes between shifting (especially from 1nd-2nd/2nd-3nd). Rarely, I will hear the noise from 3nd-4nd (and if I hear it, it will be really quiet and quick like 0,5 seconds).
By design, the 2010 Corolla transmission is a rough shifter. That's how mine has been from zero miles to a tick over 100,000 miles. All maintenance was done on time by the dealer.