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Final update: dealership techs called out SET tech because they thought it was my transmission (which they had already replaced once, before 10k miles, for crunching into 4th gear). SET tech diagnosed the problem as the direct injectors, specifically one where the teflon seal had vanished (probably burned up by combustion chamber blow-by). The pinging noise was actual "pinging" of the engine, uncontrolled detonation caused by lean running; engine was sucking in unmetered air past the injector, leading to super lean AFR in the combustion chamber.
They replaced all four direct injectors and car now runs fine without weird noises. And they're going to be getting the bill for re-aligning my car.
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