the high note and low note i think are just because the different notes carry through different mediums and having both gives a "fuller" sound

the AC turns on when you hit defrost because all the AC does is remove moisture from the air, contrary to popular belief it does not actually lower the temp of the air (it may feel like it does because lower moisture content air encourages more evaporation of moisture off your skin which cools your body, hence the reason people sweat) but so having less water (humidity) in the air will make the condensed water causing the window fog to evaporate into the air of the cabin faster