[Closed] Rattling rear hub (or cassette, or something)

Over the last few months, my winter bike has developed an increasing rattle from the rear cassette/rear hub.

Granted, I only noticed it after I had removed the cassette when I was playing with the wheels, but again tonight I took it apart, and can see nothing missing and nothing wrong with it. That said, the rattle was horrible this morning when a friend of mine was riding with me.

The wheel is a Campagnolo Khamsin Asymmetric, and the cassette is just a Shimano Tiagra 9-speed.

What am I missing? The cassette is a year old with maybe 2000 kms on it; the wheels a bit older.

Ideas?