High Pitched Whistling Noise Coming From Power Supply
If the power supply (SMPS) that is running fine and you could hear a high pitched whistling noise then suspect the switch mode power transformer (power transformer) or the secondary side filter coil (inductor) have problem. The first thing that you need to do in order to confirm whether is the transformer or the inductor problem, you use the handle of the screw driver and gently hit both the components while power is still operating (On). Try knock on the transformer to see if it will quite down or not and then proceed to the inductors. If by knocking could cause the noise to go away then you have confirm that the particular component indeed have problem.
Your next step would be to stop the noise from coming again. Dip the transformer or the inductor into shellac for couple of minutes and then let it dry for a day. Fix back the components on the next day and I’m very sure you could hear the noise no more!
I have also came across problem with dry joints in one of the small ceramic capacitor that caused the high pitched sound. Resoldered the primary section cured the fault.