A few weeks ago I found out what was causing it. It was the beginning of the "chimes of death" sequence.

Some macs play just the four chimes, but some, like the Performa 475, play an arpegio first, then the four chimes. So, what it was doing, was that it started by playing the arpegio, but that was then interupted with the normal bong. It's strange why it was doing this.

But, eventually, a few weeks ago, it stopped bonging, and played the death chimes to the end xx( . It was then that I decided to recap. It was also then that I discovered what it was doing, as the sound seemed familiar, but continued to its dreadful end.