Failing capacitors can make a high-pitched noise.
It's normal for a power supply to emit some noise while it's working, but high-pitched noise from you power supply can be a sign that you need to repair or replace it. Typically, noise can come from the transformer from a fan inside the power supply or from bad capacitors. You should always address high-pitched power supply noise as it can be a sign that your unit is failing or needs attention.
Magnetostriction
Magnetostriction can cause power supply noise. A power supply typically contains a transformer, which converts the high mains voltage into a more suitable voltage for computers and other electronic equipment. When magnetic materials such as the iron inside a transformer are placed in a magnetic field, they change shape slightly. This can create audible noise. Magnetostriction is less likely to be at the root of a high-pitched noise coming from the power supply than some other possible causes, as it tends to produce lower-frequency sounds; however, under some circumstances it can cause a higher-frequency whine.
Noise from Fans
Transformers and other components in a power supply can become very hot. Some power supplies come with a built-in fan to draw air through the unit and keep it cool. Sometimes a fan can create a high-pitched noise. This may come from the fan's bearings, in which case lubrication or adjustment may help. Dust or other contaminants falling into the fan can also cause it to squeak. Solutions include blowing compressed air over the fan, cleaning the fan with a paintbrush, applying a very small amount of a suitable lubricating compound or replacing the fan. A noisy fan can be a sign that your supply is underpowered or that your fan is close to failing.
Failing Capacitors
A possible cause of high-pitched noise from a power supply is a failing electrolytic capacitor. Electrolytic capacitors contain a fluid, called electrolyte. If the capacitor is improperly connected, exposed to extremes of temperature or getting old, this fluid may break down causing gasses to build up inside capacitor. Some electrolytic capacitors explode quite dramatically, but others, especially those with a vent embossed at the top, leak the gases through a hole in their casing. This can produce a shrill whine. Failing capacitors can damage the rest of a circuit and must be replaced immediately by a trained technician.
The "Capacitor Plague"
The "capacitor plague" was an influx of improperly-made electrolytic capacitors from suppliers in Asia. Reports of bad capacitors causing problems in electronic equipment began to emerge in the late 1990s and continued through the following decade. Equipment manufactured as late as 2007 is thought to have been affected. Electrolytic capacitors typically have a lifespan of around seven years; the so-called "bad caps" have a lifespan of under four years. Power supplies made before 2007 may possibly contain these defective capacitors, which have been known to make a high-pitched whine when fading.
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