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Sounds like the alternator. You turn on AC, noise. AC off, no noise. Seat heaters and defrost, noise. Only way a seat heater and defrost would cause issues is a change of load in the alternator. AC would use more juice as well running the magnetic latch for the compressor, and your fan.
Especially since you've changed so much already. Could be bearing in the alternator starting to go and making noise when it's experiencing higher loads. Subi alternators are known to be one of their more frequently replaced parts over the life of the car.
My 01 Outback broke alternator bolts three times, but never made any noises.
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