I also got this, was driving me nuts. At first thought it was harddrive. But narrowed it down to the PSU. Only happens at idle, just is due to some energy saving mode in the bios, need to disable it.
""We have seen that the CPU power saving features in the BIOS can cause the PSU to emit the noise. Try disabling these features to see if there is any improvement. C1E and EIST (Enhanced Intel Speedstep Technology) are two examples of power saving settings, but your motherboard manufacturer should be able to tell you specifically which features your board supports.
We have also seen that some graphics card drivers can contribute to the sound as well. If the problems seem to get worse when loading a 3D application, try booting your system up into safe mode to see if you can still hear the sound. If the sound goes away, then your problem may be software related. We have found that enabling “vsync” in your graphics drivers options can eliminate the sound."""
Worked for me!