The Campagnolo Thread

Can be either of 3 things - I would contact the Campagnolo service center (Velotech or Chicken Cycle Kit) for this as it's a common issue with a certain amount of (mostly) 9-42 cassettes.

1. The aluminium backing plate
   
2. The spacer that sits inside the cassette as a sleeve - directly positioned behind the alu backing plate
   
3. The lockring / 4 cogs combination (very likely the actual culprit)
   
   

I've tried replacing them in the above order and only the lockring /cog combo offered an actual permanent solution over a temporary improvement. I've been using the Ekar noisefree ever since.

They've implemented a running change to all three points above to ensure this problem does not occur anymore. The backingplate now comes with countersunk screws, the spacer is slightly taller and they added a thin spacer inside the lockring / cog combo that prevents the lockring to undo itself. This is the actual reason for the noise to arise.

Initially you'd think loc-tite or a higher torque upon assembly would solve it, but if you do this you'll run into problems when you want to remove the cassette, as the lockring / cog combo comes apart prior to the actual lockring releasing.

Hope this helps!