Originally Posted by
hernick
Cassettes tend to have a wee bit of play, but if you get a regular osciliation, it could be that your rear hub axle is bent. The normal wobble of a cassette looks different from the wobble of a bent axle.
Before pulling apart the freehub body, you may want to open up the rear hub and extract the axle. The bend may obvious, or it may be hard to see. The best diagnostic is to remove all cones from the axle and roll it on a glass table. Make sure to note the exact position of the cassette-side cone before removing it.
you have to remove the axle to access the freehub body anyway.
so i'd remove the axle (which is probably
not bent) and tighten the freehub body onto the hub shell. If that takes care of it, there we go. If not, freehub bodies are not terribly expensive, unless you're running an xtr or dura ace, in which case they can go for up to $100.