插上电源后键盘左下角就吱吱的发出响声，拔掉电源后就不响了！响声是间断周期循环的嗯嗯声。 请问是什么原因，求帮助！   After plugging in the power supply, the lower left corner of the keyboard squeaks and makes a noise, and when the power is unplugged, it does not ring! A boom is a muttering sound that loops in an intermittent cycle. Please ask what is the reason, please help!