BTW - I believe that the sound system is somehow involved (whether hardware, software, or both I don't know).

My reason is simple - the sound on my Powerbook 'sleeps' after a period of inactivity, and the 'buzz' goes away ... 'wake' the sound (usu. with a system event), and the buzz returns.

Now, I consider the 'sleeping sound' to be a B-U-G, and have search high and low for info about it ... but, bug or no, it does illustrate something interesting about the buzz.