Ted. -

I have a Dell Inspiron 7500 that used to do exactly that at bootup except for requiring a keystroke.  The squeal was totally independent of volume setting, but definitely was emanating from the notebook's onboard speaker.

Sometime back, I decided to "clean house" and reload all the software anew, then judiciously restore data files and utilities.  The mysterious squeal was not emitted on bootup through several iterations, but finally returned when everything was back onboard.  Then, just as mysteriously, it eventually went away several months later never to return.

I attribute the "squelching of the squeal" to frequent WIN 98SE operating system updates which likely resulted in a serendipitous patch.  I, for one, am glad that it is gone.