Hm, these things can be mysterious:
I have just replaced the entire drive train on my Campy chorus 8 speed - everything runs as smooth as can be.
Then (after maybe just a 100 miles) I decide to try the also new 23-13 Campy record cassette.
Result: double the noise from the rear, no adjustment makes it go away.
Swap the cassette with the 26-13 (also new record) - silent again. Go figure.