My computer has this problem.
Only it happens many other times, as well, including:
When pieces of a web page are loading
When menus popup
When I minimize and restore windows
When the comp is loading something (sometimes)
When there is heavy animation or flash items being shown on a web page
I have a halfway decent sound card, Ensoniq Audio PCI (which is the Soundblaster PCI 64), but I've heard that this problem is a motherboard problem for some whacked reason...
EDIT: Forgot to mension this only happens when I'm playing sound files off my HD, like mp3s, while doing the above mensioned tasks.