Best to establish that it's actually the soundcard that's crackling and popping. What are you listening through? Front panel audio or straight out of the back of the card?
Tried using the unified drivers? http://brainbit.wordpress.com/2010/07/19/asus-xonar-unified-drivers/

Front panel connector, might want to try plugging directly into card.

Make sure your mic is muted, if indeed you have a mic plugged in. Don't ask me how I know about that one![]()
Tried a different PCIE slot, if you have one spare?
Are you running with GX mode enabled? If so try it without, i found i get some crackling with GX on with my essence ST deluxe.