Packard Bell PC whining noise from PSU

Hi all,
I have just purchased a Packard Bell iMedia x4520uk PC, and am very happy with it as far as my needs go. However, one small (?) problem. After the PC is shutdown via Windows 7, I can hear a small high pitched whine/squeal from the back of the PC where the PSU is. You have to be near the PC to hear it. If the PC is left switched on at the mains, the noise stays. If it is switched off at the plug after shutdown, the noise slowly dies away after 25 seconds. But it goes immediately if the power button on the the top is briefly pressed after switch-off at the plug, as that presumeably would discharge any residual electricity in the circuits. Is this normal? Or is it a "feature" rather than a fault in the PC?
This is my second unit. I took the first one back because of the same issue, and this one does the same. And another user has reported the same issue as me and she is on her 2nd model also! It appears they may all do it! Please note that the PC does not make this sound at all when switched on (even before normal switch-on via power up switch on PC) and neither in normal operation. Hope some-one can help. Thanks.