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?