Re: "Crack" noise on either forward or backcast
Curtis is right on ... Whenever you apply to much force to quickly during either the back or forward cast, you can easily play the old game of crack the whip or pop the towel... Usually, it's the transition from the back to forward cast that gives fly fishers the greatest problem.
Make it easy on yourself and watch the backcast ... and at the transition, think "smooooooth" saving the so-called power stroke for the last. As you begin the forward cast, allow your rod hand to lead the rod until the time for speed-up and stop.