That means it's doing what it is supposed to and probably very well at that, as long as it isn't giving a burned taste it isn't cause for alarm. I find stock Kanger coils do that a lot but I rebuild my Kangers myself so mine don't tend to crackle,stock Kanger coils aren't wound tight against each other so they tend to have hot-spots in the element and that can cause the
juice to pop like dripping cold water on a hot skillet, it is very normal occurrence especially if running them on a higher powered device than say an ego battery. A 1.8 ohm stock Kanger single coil for a protank2 at 15watts will actually make a pop noise when first fired and crackle as you draw on it and it really is normal but I guess it could be a bit unsettling if you never really paid attention to it before now.