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Ahh, I recognize it, it's the ethernet interface...
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a) Fans. Fan noise is often discontinuous, "it comes and it goes", shifting from grumbling to high-pitched squealing and back again. Fans can be opened and re-lubricated, but it's not worth the effort; the bearings of a squealing fan are damaged and the problem will be back shortly. Replace the fan(s).
b) The power supply. This noise is continuous and high pitched. It's caused by delamination in the output transformer. Replace the power supply immediately, because this problem can cause the power supply to overheat and catch fire.