And the verdict - loose/bad power steering pulley. The bolts were pretty loose on it, so we went ahead and put a new one. No more sound.
Last person to touch that pulley was the dealership. They replaced it along with my subframe for the recall. That was about a year ago, so maybe that's long enough to exonerate them. Either that or they didn't tighten the bolts enough and that wouldn't surprise me.
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