Usually electronics that have no moving parts but make noise do so due to the electromagnetism inherent to current and wire that causes something metal to move.
In fact, sometimes things that *appear* to have no moving parts in fact do, insomuch as they deliberately use this magnetic effect to achieve something. For example, relays allow you to use a very low voltage circuit as a switch for a much higher voltage one by having the low voltage circuit create an electromagnetic effect which physically pulls a (hidden but audible) bit of metal such that another, entirely separate circuit is now complete. This way the two circuits remain physically isolated from each other, but this low voltage circuit needs to be maintained for the magnetic effect to hold the other circuit together. But you can hear it clicking on and off!
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