I just post this to prevent newbies in overclocking like me from doing the same thing.
In order to cool my HD, I have put a fan below it. And then 1 minute after turning on my computer, I heard a big Beeeeeepp!!. So I swiched off the computer instantly.
I'm pretty sure the problem came from the conflict betwen both magnetic forces, the one of the fan, and the one of the Hard Disk.
But thanks to Jehova my computer is apparently not injured.
Maybe someone who knows a bit more about physics & Overclocking could give his opinion on that.