There are four catalytic converters on your Boxster, two of them are a three-way bolted to the heads of the motor, the other two are further down-line closer to the muffler. Which converter is making the noise?
What might be causing the rattle noise are small pieces of gravel stuck in the heat shield of the converters that are closest to the muffler. I removed the heat shields on mine and my rattle noise went away.
As for the three-way converters there are no CARB approved aftermarket choices that I am aware of... I had to buy factory.
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