Sounds like it isn't attached to the head form very well. If you can move it back and forth pretty easy, that's likely the case. If so, I would drive small finishing nails through the fur, close to the ear, and all around it and bury them. That will fix it if it's moving that is causing the creaking. It should have clay around the ear butt to rebuild the muscle, which would have kept this from happening, but, you never know what other taxidermists do.