A new update from Creative on the crackling and popping issues some people are experiencing with their SoundBlaster X-Fi cards. According to Creative, the sound glitches only arise on nForce 4 motherboards and only nVidia is able to fix them:

We have observed through direct observation of the PCI bus on the nVidia nForce 4 motherboards that when the crackle symptoms are occuring, the Soundblaster X-Fi card PCI bus master memory requests for audio data are being held off (not serviced) for very long intervals.

We have observed peak holdoffs of up to 2 milliseconds in some cases. This is unusual chipset behavior that is beyond the ability of a hardware audio accelerator to compensate for in its internal buffering. The SoundBlaster X-Fi tolerance for these PCI holdoffs is approximately 120 microseconds peak holdoff, with a 1 microsecond average holdoff.

These design tolerances are based on observation of real PCI bus behavior on leading motherboards over a period of many motherboard hardware generations.

The nForce 4 motherboard is the first motherboard on which these extremely long PCI service holdoffs have been observed by Creative, where another PCI device in the system was not causing the holdoff.