Something could be drawing too little power. I made a 10'000 Volt power supply for my Tesla coil out of Microwave oven transformers, and whenever I draw nothing from them I get a tone from them, but if I hook up something that draws a lot of current, like a Jacob's Ladder, the tone goes away.
This is the same case with your computer. If you run a program that uses a lot of processing power, the processor will draw a lot of current.
A transformer is basically made up of two coils of wire around a metal cylinder or ring. You have a primary winding, and a secondary winding. When you put AC (Alternating Current) power on the primary winding, the electricity will build up a magnetic field around the metal cylinder (this is how an electromagnet works). Then, when the polarity of the AC current flips, the magnetic field will break down, forcing the field into the secondary winding and therefore converting the magnetic field back to electricity.
The conversion is proportional, so if I had 100 windings on the primary winding, and 300 windings on the secondary, and put 2V AC on the primary, I would get 6V AC out of the secondary. Of course, a transformer is only 80% efficient, so again 6V / 100 * 80 = 4.8V AC.
What is causing the high pitch is that when you don't draw on the secondary winding, like when your computer is not doing a lot, the magnetic field has nowhere to break down to. So the field will vibrate any metallic objects in it's reach at 50Hz, causing a high pitch sound. When you draw on the secondary winding, like when you run a 3D program that takes a lot of processing, there will not be a lot of magnetic energy left to vibrate any metal, because it is all being used for the processor.
I do not know what started to cause this, and I don't know how to fix it, but you could try tightening a few bolts and screws in your computer to see if that makes any difference.
TheComet
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