What makes a crystal oscillation to begin
"but... what noise? Audio noise? Electric noise coming from the circuit?"

Every circuit (except for superconductors. Maybe) has noise. Every resistor produces a noise voltage, as thermal energy jostles the electrons in it. It's called Johnson noise. This is why, for instance, very-low-noise amplifiers are often characterized by the effective temperature of their inputs. 

It's a direct consequence of a) the fact that charge is quantized, and b) thermal energy manifests itself as thermal motion.

There are other sources of noise as well, but that will do for a start.