Usually this perception is caused by an optical illusion. Cassettes and freewheels have angular "ramping" (leaning one way to the other) that makes it appear to be wobbling from side to side. A little play is normal. Look closely at your cassette and see if there are angular cuts in the teeth of the cogs. Then spin it fast and take a look. Because one cog is ramped this way here and that way opposite - your mind sees it and being warped.