When your catalytic converter starts rattling the rattle in the catalytic converter is because the catalytic converter is failing.
The rattle is caused by the catalyst coated honeycomb meshes on the inside of catalytic converter breaks apart or collapses as the catalytic converter ages.
This can be caused by either just simply age of the catalytic converter or from excessively rich fuel mixtures causing the catalytic converter to get too hot.
As long as your engine is running properly then driving with the catalytic converter rattling should be okay but it can eventually get worse overtime and then clog the catalytic converter completely which can cause your engine to stall out or cause your vehicles engine to fail to start.
So if you have a rattling catalytic converter I would get it replaced as soon as you can before it does get worse.