Hissing sound when turned on and goes dark when sound stops.

The cap on the coolant reservoir may be leaking a small bit of air. When the water and whatever air is in the system gets heated, it expands. As it cools later it may suck air back in, probably slow enough so that you don't hear it. When you hear the sound, press firmly on the reservoir cap to see if the sound changes or goes away. If so, then try replacing the cap. It should only cost about $10 USD or less to replace it.

You listed this as a CRT model and a hissing noise from inside the unit means the high voltage transformer is leaking voltage. Turn it on in a dark room and look for sparks

Mabe there's some air in the line. Let the water run a bit and see if the hiss stops.

The hissing sound could be high voltage leaking from a tripler. Observe in a dark room for blue arcing. Look at tripler and connections to the projection tubes. If the tripler is leaking replace the tripler.

Your AC adapter is "dying", and must be replaced.
Is it still under "warranty", i.e., you contact LaCie,
get an RMA (Return Materials Authorization) number,
they ship you a new adapter. If you ship them the old adapter,
within a week, they will _not_ charge your credit-card the $30.

If you hear a hissing sound from the gas cap then the o ring that seals the tank is either cracked or tore. the tank has to build up pressure. thats where u r hearing the hissing sound replace it, it should stop after.

I found out that if I manually turn the speakers down on the tv and then turn it up with the remote control the hissing sound goes away

The noise maybe the p/s pump fluid return,GM had some problem with this but nothing bad. GM makes a resticter that goes in the return line of the pump, it will slow the fluid down going back in the pump stopping the hissing sound.

A hissing sound that comes from the back of the set (not from the speaker) usually indicates a high voltage leak, most likely around the "suction cup" at the top of the picture tube or around the flyback transformer. Remove the back of the set and turn it on, and when you hear the hissing sound, shut off all the lights in the room and you should be able to see a purple-ish or white-ish glow from the spot that is hissing. Shut the set off, discharge the high voltage and clean and seal the problem area. Of course this should be done by someone experienced with stopping high voltage leaks.

Is the hissing or static sound comming from the speakers or from the back of the set itself?? Need more info please.