Hi Studsgaard,
Try and get hold of a Telex Airman 750 or 850 headset, they have a microphone attenuation potentiometer in the earpiece adjacent to the microphone boom, plug it in to your aircraft and tweak the headset potentiometer until the point where the hiss is 'squelched'.
Just also check that you dont have a headset in the jumpseat and the mic on hotmic setting, ive seen that causing the same problem as you describe before as well.
There are adjustments that can be done in audio control panels / audio amplifiers too to help eradicate your problem.
Good luck.