If it's not the usual chime that you get when you turn on the Mac, then something hardware related is wrong. For example, on an old PowerBook Duo 230 I sometimes get an arpeggiated tone followed by a more dissonant melody. I know there's something wrong....probably memory or something...it's an old notebook. Some systems give a breaking glass sound instead. Regardless, I know it's a hardware issue I'm having, and that might be the same on your end.