I'm trying to understand how the Pierce oscillator works. Thanks to How does a Crystal work?, I've managed to understand almost all, but I'm still struggling to grasp, what causes the crystal to begin to oscillate at the very beginning? There's still no feedback AC coming into it.
Someone says that it's the "noise" causing the crystal to oscillate, but what noise? Audio noise? Electric noise coming from the circuit?
I would really appreciate if someone could help me to understand this "instant zero" of the Pierce oscillator.