It's basically a valve with a spring on one side and gas pressure on the other. When the pressures are very close there will be a small amount of gas moving from the high to low pressure side. And the closer they get, the slower it will be. Eventually the pressures are equal, but that doesn't happen instantly. Equilibrium speed and noise surely depends on regulator design.
I've noticed that when I first turn on the CO2, the regulator pressure jumps to - say - 8psi. When I check after a day it will be a little higher - like 9psi - and stay there. This is my explaination for that.
I've read regulator instructions that say to start with the regulator at zero, turn the CO2 on, and then SLOWLY turn up to the pressure you want. Maybe this is why.