I've not personally ever heard of any problems with cassette carriers/spiders, but along that theme, maybe some of the rivets that hold the cogs on have become loose? That might cause some creaking issues. I know chainrings can creak if the bolts come loose, so maybe cassettes can too.
There's just a lot going on back there and there are a lot of potential noise sources (i.e. many mechanical interfaces, bearings, pivots, spoke-to-rim and spoke-to-hub junctions, etc). Maybe the freehub is a little loose or something as well. Maybe the cleaning doesn't really do anything but the act of removing and reinstalling the wheel re-seats the axle or bearings in some way that makes the noise go away for a while.