Re: the disappearing fly - super newb
The minute I read this "the disappearing fly" I knew what this thread was about.
That crack sound you are hearing is just like what a whip would sound like because your line is doing the same thing.
Basically you are starting your forward casting stroke before your backward stroke is finished. So your line is starting to unroll behind you and moving at X speed, then you start your forward stroke which accelerates the line even more as it unravels, and the mass of your fly at the end of the line sends it spinning off into space.
In the case of a whip the "Crack" is actually a small sonic boom as the end of the whip goes supersonic.
In the case of your flyline/fly I suspect that may also be the case, but I guess it could just be the sound of the line breaking as your fly goes off into outer space.