Same here, and I don't like it. Mine, I would say, are after an
"unproductive" blowing of the nose. To me, it sounds like stuff that's
too high up and/or too thick to come out, maybe with pockets of air
trapped in or around it. It sounds like the stuff is trying to come
"unglued" from the walls of my sinuses, but can't. I can feel the
congestion, and would suspect that's what your's is too.

I'd love to hear a more effective way of breaking the stuff down, so
that maybe it could get out. Even the CanDo method seems to get me
more of that creaking noise by breaking the stuff down some, but
adding to it and not getting it out.

                                                - Good Luck

That's the negative pressure (vacuum) within the sinuses, which is
created by the swollen turbinates and mucosa, sucking air back into
the sinuses after you blow your nose. I had the same thing, that's
what the doc said.

I'm having surgery in two weeks. That should prevent that from
happenning anymore. Yay!