If you're stuck, and since they're so old, it might not be a bad idea to replace the bearing. Process of elimination. It's not great to throw parts at an issue without knowing what it is first, but it seems that it may be your next move in all this. Those are all the tests I would have done to confirm a bearing, but then again I too was unsuccessful when doing some tests to determine if my bearings were an issue. I've never heard CV's squeal before as their typical noise is a click. Anyways, posted them below if needed and if you decide to replace them. Let us know what happens!
2003 Porsche Boxster Base Convertible - Axles, Bearings & Differential - Page 5
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