The X-Fi generated crackling noises from 40% (2/5) of the motherboards. I’d consider it a high number because PCI cards are suppose to fit and work fully within the PCI specification. If this card fails to work on more than a single motherboard, the manufacturer should investigate and do something about it.
If this article is not enough for you I suggest to head over the official Creative support forums and do a search for "SCP" which is an user-made acronym for "Scraping/Crackling/popping." Be warned that the number of hits for that word might make you think twice about buying a X-Fi soundcard, just like me.