Guys!
I fixed it. Unbelievable...
It was another power distributor that caused the noise. It was plugged I to the same one the Kemper is in, but that’s not the only problem. It doesn’t matter where I plug it in! As soon as I unplug it the noise is gone, I plugged it in somewhere else, it’s back. So it seems that this, somehow faulty, power distributor fucks up the whole electricity net in the apartment.
Thanks a lot for your help guys, much appreciated!