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If no error pops up and if no applications from the dock are bouncing, then you may have a background application making that noise to either confirm an activity completion (new Mail, KDX error message (KDX is an application from www.haxial.com ), a background render is complete, etc.). Sometimes you can undo this setting, depending on the app and its preferences.

Also, do you have sufficient battery life? If not, your iBook may be beeping for this reason.
 
No, the 'Book was plugged into the wall at the time. I was just using iFlash (a flashcard program) and iTunes...nothing else that I know of. I just ran "top" in the terminal and I don't see anything out of the ordinary...
 
From what I have ascertained, powerbooks, ibooks, etc. will beep (with or without volume) anytime you do something that improperly flexes the case or disturbs the inner workings of the machine. As everything is pretty tight in there, it is relatively easy to do. (Happens more frequently on my Ti than on my iBook.)

Although I will qualify this as experience based info and not technical based info, as I have experienced it many, many times, I have noticed that the trend matches the above reasoning. In fact, if you tap the trackpad (or next ot the trackpad) too hard on my TiBook it will beep for the same reasoning.

Can anyone TECHNICALLY confirm this?
 
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