The good old Nforce 4 and X-Fi pops and clicks. Good luck getting it sorted as I don't believe that that problem ever really got fixed by anyone that had it short of getting a motherboard that didn't have a Nforce 4 chipset on it. For those that don't know about it this has been going on with that particular chipset and X-Fi's (and most other sound cards as well) since it came out,

nForce4 - Wikipedia, the free encyclopedia

The nForce4 chipset has also been blamed for issues with PCI cards, relating to Nvidia's implementation of the PCI bus. RME Audio, a maker of professional audio equipment, has stated that the latency of the PCI bus is unreliable and that the chipset's PCI Express interface can "hog" system data transfer resources when intense video card usage is occurring. This has the effect of causing audible pops and clicks with PCI sound cards.[7] Gamers have noticed this effect, especially with Creative's Sound Blaster X-Fi and Sound Blaster Audigy 2 sound cards. Compatibility issues between these sound cards and nForce4 motherboards have been ongoing, even following driver updates.[8] Latency issues are more readily apparent with sound cards than other addon cards because of the direct user feedback the audio problems demonstrate.
and lots of results about it here,

nf4 x-fi - Google Search