The crunching sound is most likely the rust inside the door bottom, although there is a chance the thread in the door top has let go. They may be Rivnuts/Nutserts.
What ever you do, don't prise up the door top, you'll destroy it.
I would (remove the glass from the frame first) then cut off the nut/s at the bottom of the post/s and then drift out the rusty posts. Then you can see what's happened in the door top. If necessary the threaded inserts can drilled out, the bottom frame can be built back up by TIG welding the hole and then re-drill and threads restored or insert a new Rivnut.