If it wasn't Crackling a few weeks ago then somthing has changed in your system. Perhaps you have installed a program, or piece of hardware that is conflicting with the sound card. Or, your speakers are finally begining to die on you, or your sound card is dying on you. Could be anything. Work through this checklist for trial and error.
1. Check all connections at the back. Make sure there are no loose connections.
2. Try a different set of speakers on there to see how they work. If you can, hook up ya hi-fi speakers to it. Through the stereo unit of course.