My Samsung TV has been making a hissing noise for a while now, and it is only getting worse. It only used to be present for the first 10 min of the TV being switched on. Now it is hissing all of the time.

Putting the energy saving mode onto high gets rid of the noise completely but makes the picture far to dark to watch.

Has anyone got any advice on what could be making the noise and how to fix it.

Assuming it's LCD and conventional ccfl backlights, then there's a problem with one of the tubes or the wiring.

It's surprising it's still working, many sets sense such arcing or corona discharge as a fault and shut down the backlight inverters.

Another fault condition that can cause this if sets with a master / slave inverter, certain fault conditions means they don't work in sync with each other and make that noise, but again, this is usually sensed as a fault and they shut down after a couple of seconds.