When a catalytic converter starts to rattle the rattling of the catalytic converter is usually an indicator of a failing catalytic converter.
Once your catalytic converter starts rattling it means that it's not working properly anymore and needs replaced soon.
You can still drive the car or vehicle with a rattling catalytic converter but if you have to get emissions testing on the vehicle then the vehicle will fail the emissions testing.
So you'll need to replace the catalytic converter if you have to have emissions testing done.
The cause of the rattle from the catalytic converter after it fails is because the honeycomb meshes inside the catalytic converter collapses and breaks down causing the rattle of the catalytic converter.
If your state or county has no emissions testing then you might be able to just remove the catalytic converter and do away with it.