Computer makes sound when its shut down

My rig :

asus p5b
e6600 @ 2.4ghz
2gb memory
320gb sata samsung
520watt powersupply corsair
7600 se graphics card

The problem is when the computer shuts down after like 5 seconds ( when the computer is shut down ) there comes a weird crispy sound , which hangs around for 30 seconds then goes away and 15 sec later it comes again for like 15 seconds . ( then it stays away )
I am 100% sure it is not my processor/cooler/harddisk , it is comming from the ICH8 section which is above my sata ports .

The PC is 2 years old and I am having this problem for 1 month now .
My computer works fine , and I am not sure if this can be any harm .
I tried to google this problem but no one seems to have it , I only get sound problem issues .

Help is greatly appreciated .

Please help me I got the same problem,but its worse.I bought a Intenso Video Traveler 4gb mp3,and when i plug it to the computer it makes the sound and shuts down after a few seconds,please really need your help,because i can't recharge the mp3. .

My configentium4 3.2ghZ
Ram:1.5gb
I don't know where to look for my power supply the Everest doesnt show anything.