Originally Posted by DRLski
turning the adjuster barrel doesn't do much for the noise, it's shifting crisp. The derailleur doesn't "look" bent to me. It sounds like it's coming from the actual cassette...unfortunately I don't have a spare wheel or cassette to test the theory.
The cassette itself doesn't contain any moving parts. If you grab the offending cogs with your fingers, can you wiggle them side to side or front to back at all?
The cassette is attached to the freehub body, and thereby to the hub itself. I suppose there could be something wrong with the hub bearings, though not sure why it would only be noticeable on some cogs and not the others.
If you take the wheel out of the frame, and spin the axle by hand (leaving the rim stationary), does the axle feel smooth?