I would think the only thing that would make rattles stick is that water has gotten into the bait, and rusted them.
Before you go through the trouble of unsticking the rattles, make sure that there are no leaks at the hook hangers, or the bill. You can use runny super glue to reseal and strengthen those attachment points.
Then I would drill a small hole in the top of the bait, and use a wire to find the stuck rattles. Enlarge the hole enough to get the compromised rattles out, clean out any rust that you find, and put some sst balls in as replacement rattles.
Then seal the hole with bondo, coated with runny super glue after it's been sanded back down to flush.
Repaint or touch up as you like.