Check your motherboards southbridge temperature, distorting or crackles in audio can be one of the first symptoms of the southbridge overheating. Basically feel it with your finger, if it feels red hot then try adding some more cooling to the PC or even better add a small heatsink to the southbridge as well.
I learn't this the hard way earlier this year: my audio started to get a low sounding hum/distortion if you listened carefully. I narrowed it down to the PC as disconecting the coax to the reciever made it disapear, but changing the cable did not. It was awful as when in a film for instance there was the sound of traffic there would be this terrible amplified distortion that I never used to get. It got worst though, the memory controller started randomly failing and the computer would just restart at the drop of a hat. Then I get after a restart: "IDE Controller Failure". The motherboard never worked again. Apparently the problem is quite common on Nforce2 motherboards like the ASUS A7N8X-E Deluxe I was using, old but nothing beats the onboard Nvidia Soundstorm with on the fly Dolby Digital Live encoding.
With my new motherboard I am going all out on cooling to prevent a similar occurrence.
