More often than not it's the "Buffer" setting. Your buffer setting is probably a bit too low. Go to "Options," then "Audio Settings." Try moving the slider up to the right until you don't hear anymore crackling. Like Carnage187 was also saying, it could also be because your CPU power and Ram ain't up to par with the amount of FX or samples your trying to use.
Another thing, "ASIO" driver is a must and it helps achieve lower Buffer settings for lower latency, thus getting quicker response from your synthesizer, fx plugins in realtime and overall just better, cleaner sound when playing back inside FL Studio. The lower buffer setting you can get without crackling playback, the better.
Download this program "ASIO4ALL" off this site (
http://www.asio4all.com/ ), install it, then go to the "Audio Settings" of FL Studio and then under the "Output" section, selct "ASIO4ALL v2" from the little dropdown menu.
Then now go hear your song and see if it still crackles. If so, go back to that audio settings, then under the "ASIO properties" section, click on that box that saids "Show ASIO panel," then once that ASIO program pops up, move the slider more to the right little by little until all the crackling is gone. That should definitely help.