Some answers to your questions from a fellow PB100 owner:
1) It is perfectly safe to plug in the AC adapter anytime you want to, even
when the computer is on. As for the modem/printer cables (both of which are
plugged intot he printer serial port, I believe) I have not found anything
that says you shouldn't. I routinely plug a LocalTalk connector into the
serial port when the computer is on, so I can go print on the office
LaserPrinter. I have experienced no problems. I DO know that on a desktop Mac
it is OK to plug/unplug items from the serial ports when the computer is on,
but that it is NOT OK to plug/unplug ADB devices (e.g. keyboard/mouse) when
the computer is on. I would say the same thing for the PowerBook.
2) Strange beeping sound? I have noticed that from time to time my PB100 will play the system beep when I open a file or switch to the Finder from an open application,
but this I have attributed to one of the NOW Utilities I am running (Super
Boomerang, maybe?) I never thought much of it and anyway, you mentioned that
it was NOT the system beep but something different. I have no idea what could
be causing it. Does it do this when you are running with all extensions off?
3) When you rest your hands on the keyboard of a PB100, your left palm is directly above the hard drive and your right hand is directly above the battery.
As you mention, I also have noticed that the surface is often slightly warm to
the touch. My best guess is that it is caused by (harmless) internal heat
generated perhaps by the battery or the drive, and this was a purposely
engineered feature by Apple to keep PowerBook users' hands warm while typing
in cold climates. :)
Hope this answers your questions. Let me know if you have any more. (Or if you
come up with more info on your "strange beeping sound."
