NATLANG: What's the sound of Castilian <s>?

I have a coworker from Pamplona and I can't quite make out the
sound of his <s>es when he speaks Spanish.  It's definitely
"softer"/more palatalized than [s], but I don't think it's [S];
it might be [C].    I was wondering if anyone on here knew what
it was most likely to be, assuming his speech is typical of that area?

-Mark

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