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I know generally cats can rattle if they go bad, but not sure about the Boxsters. Bang on yours with a rubber mallet, who knows? If your tensioners were worn, or your pads I think it would be odd to only hear the noise at a single rpm range and not at all on cold start. For giggles, check your rotating things. PS pump, alt etc. it might be that your alt has bad bearings and the change in rotation is making them more known, or possibly the alt pully. They can go bad as well.
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