The casing, commonly called the "cassette body," is held to the hub shell by a hollow tube-shaped bolt that's tightened/loosened by means of a 10mm hex key. One possibility is that this tube-shaped bolt simply isn't tight.
Another possibility is that the cassette body is fastened to the hub shell properly, but the cassette body's own bearing system has excessive play. If that's the case, and it's a Shimano, and you're the original owner, it should be possible to get a new cassette body under Shimano's warranty, provided you didn't abuse it or something. These can
be taken apart and a shim removed to reduce their bearing play, but it's not something that I've seen in practice much.
If the cogs are all snugged together and don't have play among themselves, then that would point to one or both of these other possibilities. On the other hand, if the cogs are not being compressed enough to keep them snugged against eachother, even though the lockring is tight, then you are apparently up against the problem that your LBS tried to fix already.
At any rate, if you bought the bike new, then your LBS should have the green light to warranty the whole wheel if necessary (barring neglect/abuse). Good luck!
I guess I'm assuming that this is, in fact, a Shimano rear hub. There are plenty of non-Shimano-brand hubs in use out there as well. More info on precisely what parts you've got wouldn't hurt
Last edited by mechBgon; 06-05-03 at 01:03 AM.