Hi, FWIW
I have been reading this post with great interest..Why? Because I have heard the noises you describe many many times before in many devices not just computers..
In this case "it is the nature of the beast"..Here is why,
To correctly describe it "it is acoustical noise rather than Electrical noise..
Let me do some background first..You all know that Computer power supplies use switching transistors/FETs to generate the required voltages needed efficiently rather than a power hungry analogue regulator..Thats how they can achieve 80% numbers..
A square wave is made up of a fundamental frequency and odd harmonics(3x,5x,7x etc) of the fundamental frequency..
A non rectangular wave consists of both odd and even harmonics of the fundamental frequency..(2x,3x,4x,5,6x,7x etc)
Many power supplies employ both varying fundamental switching frequencies as well as duty cycle variations to generate the +12,+5 +3.3 and so on..Both frequency and duty cycle depend on the load as it varies..Each power supply is differently designed..
In addition the mother board itself employs some power regulation..An example of that is generating the Vcore voltages (usually from the +12 volt supply)
Now you ask ,So What..Imagine if you will if you could see these harmonics moving up and down in both amplitude and frequency as the load current varies..Side note A Spectrum analyser does just that)
The next concept is to understand that you have to change these varing pulses back to some DC voltage..This is usually done with some filtering, and that is usually a LC circuit (Inductor capacitor) that you can see on the board and if you could look in the power supply you would also see them..
Usually located by some thick magnetic wire around some core material.. Here is your culprit 90% of the time.. Each coil has some resonate frequency based in it's design and when a harmonic excites this resonate frequency if conditions are right it will "sing".. The magnetic flux increasing and decreasing caused the core to vibrate at the resonate frequency in the audio range..Similar to a piezo buzzer effect..
This is why "Tweaking" sometimes will mis align the harmonics to stop the noise only to reappear at some different condition..
It is a very elusive problem and not always repeatable ..Designers and engineers try to minimise this effect by either a conformal coating or even by potting the inductor in some enclosure..Thats not always successful..You can testify to that..
The solution is also as elusive as the noise itself..If the sound is coming from within the Power supply you have two options,either tweak the load or RMA the PS..
If coming from the board itself you have the same above two options or sometimes if you are lucky take a thin wooden stick about a foot long carefully and gently touch each inductor..If the high pitch noise stops you have found the culprit..This is hit and miss because you have to be lucky enough first to hear it and also locate the individual inductor..
If located I suggest you go to the mfgr. and tell them you located the "singing/squealing" inductor and what do they recommend that will not void your warranty..(get it in writing)
Each and every case is different so this post may not fit your case,but just maybe it may help understand and even fix your problem..
This post is meant for those who do not understand and I am sure that people with an electronics background will have run into this long ago..
Good Luck,
Jolphil
BTW One of the benefits of age is tone deafness.. I can't hear them any more..Haha
PS: I must confess I did not read all 44 pages each and every post so if this was explained..I apologise
