>> And why does even the electronic ones 'tick'?

Well, the original engineering proposal was for a simple LED indication
of "armed."  There were some worries about daylight visibility of LEDs,
however, and by the time marketing was satisfied we had implemented the
three-speaker 3x-oversampled digital dolby surround sound implementation
of "ticking" that you see in the final device.

 :-)
BillW