Because the metal conductor cuts the magnetic lines of force to generate an induced current; in turn, the magnetic lines of force pass through the metal, and when the magnetic field is constantly changing, there is an induced current in the metal conductor, called eddy current. This is a loss that eventually heats up the conductor.
The transformer adds the primary and secondary coils to the iron core, and the magnetic field generated by the primary coil changes in the iron core, and the secondary coil generates an induced voltage as an output voltage. However, the iron core itself is a conductor in which a change in the magnetic field causes eddy current loss. To increase the efficiency of the transformer, eddy current losses must be reduced. Therefore, the iron core cannot be made of a single piece of iron. Instead, it is made of a piece of silicon steel sheet. The pieces are stacked into iron cores. The sheets are basically insulated. The resulting eddy current is limited to each thin silicon steel sheet, and the eddy current loss. decrease very much.
It is precisely because a piece of silicon steel sheet is laminated into a core, under the action of an alternating magnetic field, vibration occurs between the silicon steel sheets, and a "click" sound is emitted.
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