I'm NOOO expert, but I think I've experienced this before. My observation I have noted is that if you pull the braek a little, it will put a small amount of pressure of the rear disc brakes, and if you push the car, or if it is on an incline, the breaks will creak. I asked my mechanic brother because I thought I had a problem, he told me to "yank the crap out of the break, I can't hurt it." So now, when I pull my break, I PULL my break. And I regularly park on a 20-30% incline and it doesn't creak anymore, I yank it.
On the same note, let me encourage you to pull your brake and leave your car in gear. I work in insurance and have heard/seen multiple occasions of manual geared cars rolling away, one time into a house!