My Roadster at 15,000 kms (9,300 miles) started making rattling noises from underneath, so booked it in for them to look at, and they say the Catalytic Converter needs replacement, the internals shrink, causing the rattle.
They are saying to replace the one that is rattling, they have to replace 3 of them, (there are 4), I guess as they come as a whole piece, and the whole job will cost around
AUS $2,000, (US $1,500), but it is under warranty.
The Service manager (who is American), was saying in the States, the exhaust warranty is 180,000 kms, but here it is only 100,000 kms, the same as the rest of the car.
The car gets mainly long drives.