So when I put the vacuum at more then 12% or so, it starts making this constant hissing sounds wich doesn't go away. Is this normal or should I disappear after a while?
Not sure if it is normal. However, you can turn the vacuum pump switch to 0 to stop the sound. Just be sure to put it back on when you want to release the brakes.
I'm guessing here, but I wonder if the hissing is because you have to maintain the vacuum so it constantly sucks air from the pipe?
Then you should also hear the sound when the brakes are released. You need to mantain vacuum to keep the brakes released. You then increase the pressure to apply brakes. If it would be the other way around - creating vaccum applies brakes, then a brake duct rupture would mean that you're no longer to apply brakes. This is actually how early vaccum brakes worked, and it resulted in exactly of these kind of accidents.