Did you fix the underlying problem that caused the cat to go bad in the first place? They NEVER go bad on their own. There's ALWAYS an underlying problem. Read this post on cat converters

http://ricksfreeautorepairadvice.com/cat...

You have a nice story but it is about impossible to answer a question of a loud noise.

You are assuming that this new problem is linked with the earlier replacement of the Cat and there is absolutely no reason to assume that at all

Get it fixed right away

Not even a month ago my check engine light came on on my 2003 hyundai elantra. I took it to get checked and it was the catalytic converter manifold so I bought the part at auto zone and took it to my mechanic he fixed it and the car ran great until two days ago the check engine light came back on and now my car is making a loud noise when idling or when I press the gas. I have warranty on the cat and hoping I could get this fixed soon. Any advice on the subject would be greatly appreciated I don't know much about cars.