I have a 1999 Audi A4, 2.8L, FWD. I have a check engine light and the code says that both Cats are below efficiency, bank 1 and 2. I understand that when the cats go bad they start to rattle. My cats just started to rattle after a 5 hour drive. Should I replace cats and 02s?

 

They don't need to rattle to be bad, but when they do, they are REALLY bad.
Just replace the cat converters.

 

Sometimes when it says both banks are set together (as in bank one and bank two are bad) theres a big possibility something else is causing the code to come up. Such as fuel issues, air metering issues and so on. Just keep that in mind before you dump a lot of money into it