I think a part is actually missing. That part I described above has two hoses running from it, as well as one more hose that leads to nothing. I checked within it's radius and can't find anywhere where it could have possibly been connected. So I took some gum and plugged the hose, now there is no more hissing. The penny-gum cover I made is obviously still sealed. So this hose must've been connected to something before. The car still heats up though.
No, I don't speak fluent German. I'm actually American. Yes, I agree that could be the reason for my treatment, but also because I have an old car.