Yes, the crackling is absolutely normal and I would actually be more concerned if there wasn't a crackling sound. What you are hearing is the coil heating the liquid and turning it into vapor. Much the same way water droplets or bacon fat will sizzle on a hot skillet, e-liquid will sizzle on a hot coil. If the coil wasn't crackling at all, then chances are you have a bad coil.
It should be noted that e-liquid, airflow, the particular coil and the coil resistance you are using can all have an effect on how much "crackling" you hear. For instance in my Kanger tanks I get a lot of crackling, in my Aspire tanks, I get a little less. In my RTAs and RDA's, where I am building and wicking the coils myself, I get the least crackling. E-Liquids that are higher in PG seem to crackle less for me than liquids that are higher in VG, most likely because the PG is thinner and will burn more efficiently. Everybody will experience something slightly different based on their personal vaping preferences, though.
Which kbox and which istick are you using? Having that information will improve the recommendations. Without that information, though, I would start at about 35 watts and bump it up half a watt at a time. As you bump it up, you will get closer to your "sweet spot" (that point where you like the balance between flavor, vapor production and the vape isn't too warm or too cool). You will know when you get past that sweet spot because the flavor won't be what you like, the vapor production won't be what you like or it will be too warm (or a combination of all three). Once you hit that point, back it off to the last spot you really liked.
Basically, as long as you adhere to battery safety there isn't a specific wattage you should be vaping at. The right wattage is whatever wattage creates your sweet spot, and that is different for everybody. For some of us, that wattage may be higher because we like a warmer vape, for others it may be lower because we like a cooler vape. And for many of us, we will find that we run the exact same tank with the exact same coil at slightly different wattages based on the e-liquid we are vaping at the time because different PG/VG blends and different flavors react differently at different wattages.
