2008 Other Mercedes Benz Models Catalytic Converter?

I purchased a new 2008 mercedes c300 (3.0L V6) about 2 weeks ago. Within the first week (about 250 miles) the "check engine" light came on. Therefore, last week I took back to the dealer who kept it for 3 days.

I picked it up yesterday. The service advisor told me that the engine light was caused by the catalytic converter and thus they changed the catalytic converter.

First is this even remotely normal with a new car? It has never happened to me before.

Second, since picking up my car, I noticed that it is not as smooth as it was before. It sounds like a gargling noise and is louder than before. On the plus side, the car seems faster and much lighter on its feet. It ran smooth and thick before, now it runs louder and faster.

Is there a way to return the car back to running smoothly? Will the new converter hurt my car in a any way? What would you do to make sure the new converter was installed properly?

I am sorry for such a long message and I sincerely appreciate your help.