it sounds like a short of some kind or a bad cap discharging. the noise is most likely a residual current being shoved through from some cap that discharged late on your MB.
visualy inpsect your board and sound card for any burst caps or leaking caps. even solid caps can burst/leak as i dont know of any MB's that truly have solid caps in the solid sence. its all a marketing gimic, but thats a discussion for another time.
try and force this to happen on your comp, see if there is anything or any combination of things you can do to make this occure. perhaps playing lots of music while watching videos or maxing out your CPU/video card.
if it is a bad cap i would expect this to happen when the comp is idleing along doing something small but steady then gets a large power request. I would test this by setting up rthdribl and prime95 and just let them sit ready to start there tests. play music for 15 min or so then start both of them at the same time. this will max your CPU and video card power draw, if the comp shuts down its a bad cap some place, could even be your PSU. start swaping parts and see what stops it.