Yes, sadly, the others are right. The hissing was the dying gasp of a CRT whose vacuum-seal nipple had just been snapped off. It is now an ex-CRT, but a beautiful objet d'art still. It can serve in poignant testimony that nature abhors a vacuum.

If you attempt to power up the Mac in its present condition, the high-voltage power supply will struggle valiantly against the loading by vast quantities of conductive, ionized gases. A beautiful audiovisual multimedia pyrotechnic display will soon be supplemented by a strong olfactory component, and then, ultimately, by extreme badness.