What you hear here is the Barkhausen quantisazion or -distortion. It's simply a threshold function determining the minimum-energy-level transferable because of ferromagnetic "cell" or crystal size, and the related energy content required to flip these. The better magnetic conductivity, the smaller crystals and the longer the "tails" of natural-room reverb..
If you want to hear something extreme in this direction, try a nano-crystalline or amorf-core mic stepup transformer - to me that was an almost scary experience
/Jakob E.