Hub Cap Noise

I have a 98 Crown Vic with the stock Ford Hubcaps on it. The caps seem to make alot of noise as if there is a steel bearing rolling around inside of them and it cant figure out what it is. Sometimes they will do it, sometimes they will be quiet for a few weeks. I've tried loosening them, putting them on tighter, adjusting air pressure. Nothing seems to work. Does anyone know what might possibly be causing this? And its not just one, its all four.

When the retainers lose their "point" and get dull they start to get noisy. Filing the retainers to a sharp point will help a lot, but the only true cure is the late model covers.