[all variants] noise during first stages of boot

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http://ubuntuforums.org – hi, when I boot my desktop, it's making a loud noise during the first stages of the boot(probably the POST). It's a noise like a fan that isn't running smoothly, or a HDD with serious problems. But when GRUB appears, the noise becomes more silent, when gdm is loaded the noise is completely gone, and I never hear it again, not even under heavy io or cpu/gpu load. The noise is typical for a skinning Does anyone know what could be producing such a noise during the POST? I would like to have an idea before I open the case. Thanks (Hardware)