I am trying to repair the power supply of the sewing machine. This card I think, which is in SMPS structure, is an LCC Resonant feeding card. I have repaired this board and I get the output voltages healthy. When I plug in the 220 AC sockets from the card's input points, there is a buzzing sound from the small transformer. When I searched a bit on the internet, I learned that they called it "coil whine". The system works well, but this noise gets even more severe, especially when the card's output pins are open. I am affraid of exploding this power board.
I added a video (the video link) to better explain what I wrote above.
in the first case:
card's outputs are open and energized. There is a noise,
in the second case
When I put a magnet on the transformer, the sound is cut off. I saw this in a video. However, this is decresing the output voltage 24v to 12 volt. I think this transformer goes to the mosfet's Gate.
In the third case,
I plugged a fan into the 24 V outlet and again the noise was gone and the system works healthy. What is the reason for this noise and how can i fix it?