I have a bike that I bought second hand a year and half ago, I immediately replaced the wheelset with Ksyruim Equipe's and used the like new HG CS5600 cassette that came with the bike. I replaced the 105 crankset chain rings with new 105's and a new Ultegra chain. I assembled and adjusted everything, it road great, no issues. My daughter took it to college and appears to have put a few miles on it. Fast forward to a few weeks ago when she brought it home during spring break, while on a ride I could hear what I thought was a out of adjustment rear derailleur. A couple of times during the ride we stopped so I could attempt a fine adjustment to the cable tension, regardless of the direction I adjusted the noise would not go away.
Tonight I took the cassette off, thoroughly cleaned it, reinstalled and torqued to spec. I took the chain off cleaned and oiled it, removed the pulley set, cleaned and lubed, then reinstalled. I verified the derailleur adjustment when the wheel was reinstalled, then crossed my fingers and hoped for the best only to have the noise still present.

While checking everything, I discovered the individual gears of the cassette can be moved while on the hub, apparently the noise is coming from the movement of the gears. There is a spacer installed prior to the cassette being installed, I don't recall if I put it there because it came off with the cassette when I removed it from the wheel that came with the bike.

I am at a loss. Any ideas?