My old suitcase had the same rattle for a year before a wheel snapped off at the airport. It's wild how we ignore these small, fixable issues until they cause a real disaster. What's the next little thing you're putting off fixing?
Have you popped off the little plastic cover on the wheel housing? A lot of times the screw is under there. If you can get to it, just tighten it with a screwdriver or allen key, whatever fits. Sometimes the screw itself is stripped though, and tightening won't hold. In that case, a tiny drop of strong glue on the threads before you screw it back in can save you, lol.
How do you keep glue from getting on the wheel so it still spins? I tried super glue once and it glued the whole part shut, which was a bigger headache than the rattle. Is there a trick to getting it only on the threads, or do you use a type of glue that doesn't seep into everything?