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What's wrong?

For the last couple of days. While my car is sitting still, running. I’m hearing this crackeling sound under my car. Around the middle. Sounds like metal balls just rolling around. Claning. But not real loud. Sometimes it sounds like its coming from the exhaust.

Your catalytic converter might be broken up internally. The monolith might have broken loose from the shell.
If your car still passes smog, you should have no legal reason to replace the cat at this time.
Get it checked out by a competent mechanic. It sounds like it might be easy for him to locate the cause of the noise.

Could be a broken heat shield around the catalytic converter.

@Terena, if yuu or a friend has a car jack and jack stands, put one side up on the jack stands and crawl under the car to see if it is the catalytic converter heat shield. An exhaust hanger might also be damaged. Just jiggle anything that looks like it’s not part of the body and see if anything is loose.

DO NOT DO THIS ON A CAR JACK. IT IS UNSAFE. I apologize for “shouting”, but your safety is the most important thing. You must use jack stands or a commercial lift. Doing this yourself or with your friend will give you a better idea what is wrong when you go to get it fixed. Maybe you can even fix it yourself.