The click is the power manager shutting down the amplifier to extend battery life. It happens approximately thirty minutes after the last sound is played. It's not a bug, but it can be annoying.
For the PowerBook 100, 140, 145, and 170, you can disable the speaker by inserting a mini plug in the sound out port. When you want sound, unplug it. This way you can eliminate sound and turn on the system in a meeting without disturbing others.
For newer PowerBook computers (the 160 and newer, as well as the Duo series), if you want to shut off sound entirely, set the volume in the Sound control panel to zero and the PowerBook will not make any sound, at startup or any other time.
Article Change History:
15 May 1995 - Updated for clarity.
10 Aug 1993 - Rewrote for new models.
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