that hissing is the Iron Core Chokes used on the board. and while it is extremely irritating (i had a power supply that started to do this) it will not harm anything in your system, nor is it any risk of failing.
To explain why this happens, the iron core choke is basically a iron bar with copper wire wrapped around it. you can see them in some really old boards that look like this http://www.electronics-project-design.com/images/InductorCoil.jpg
if the wire is loose at any point of the coil it can vibrate at extremely high frequency and resonate creating this whining sound (sometimes this can be heard as a hissing sound)
The iron core choke was then surrounded in epoxy to help dampen the vibration of a loose coil and they became like this http://www.gigabyte.com/microsite/48/tech_080924_ud3_ultra-d-05.jpg
Now that your educated in what a choke is and why the noise is made you can locate your motherboards chokes and attempt to stop the vibration. the best way to do this is with how glue or candle wax. but first you need to locate the choke/chokes that are causing the sound.
to do this you need to have you computer on and loaded so the noise is audible. with a plastic ball point pen, use the end without the tip to touch each choke. be careful not to touch your board however with your hand as you know this will not end well. when you touch a choke that is vibrating the noise should dampen or stop.
after testing all the chokes and you have located the problem use hot glue or wax to drip a decent size blob on the side of the choke and the motherboard so the blob will connect the two. this is all you need to do. this may stop the noise completely, or it may just soften it. youll have to do it to find out.
Tip: remove the motherboard before dripping glue/wax on to it,