Hi Richie,
The coil in your car is really just a small transformer. That noise your hearing is common to transformers, the proper name of it is Barkhorne if I remember correctly. Ever walk outside, especially at night you can hear the pole mounted electrical power transformers humming. Due to the cyclic magnetic forces and forces that exist between parallel conductors carrying current it is necessay to clamp laminations and impregenate the coils inside the ignition coil. Insufficient clamping usually results in hum. Another cause for hum in transformers which is most common is due to magnetostriction, which is the continued expansion and contraction of the transformmer steel due to the incresing and decresing flux in the core. This information just touches on why the hum is present, there still is more explantions that has to do with domiain boundaries and inpurities within the mtal of the transformer. If you don't understand anything about transformer then I can explain if you like.
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