What is this sound?

Gurgling

Too much liquid in the coil or battery is low. While holding the fire button, blow into your vape for 3 seconds, or charge your battery.

Popping

Coil may have too much liquid in it. While holding the fire button, blow into your vape for 3 seconds.

Crackling

This is a good sound ... it means your batteries are fully charged and your device is vaporizing liquid adequately.