2006 Hummer H3 Catalytic converter

Actually you have nothing to worry about. The converter on the car falls under a (minimum) 5 year 50K mile warranty. This should be covered 100% by the manufacturer.

Contact the dealer and remind them of this. THey may be trying to "double dip". You pay them and they get money from GM. I'm not saying they are, but I know it happens. ALso, make sure the they scan the computer to make sure that is the only problem. All emissions components are covered.

Check your warranty manual. Some of the newer cars even have a longer emissions warranty.

Does the rattle occur at about 1900 rpm when torque braking or accelerating under load? There is a small heat shield at the elbow where the exhaust manifold comes down and meets the first connection. This shield usually buzzes at about the speed mentioned above. Often times you can tweak the shield a bit with a prybar or screwdriver, and get it to go away. As far as the check engine light, it is probably unrelated. You can take the vehicle to a parts store, and they may be able to run a scan of trouble codes for you. Your converter is actually covered longer, because it is an emissions item, at least 80 to 100k. Realistically, the dealer can do the diagnostic for you, as it is under manufacturer warranty(bumper to bumper).