After a forehead recontouring there is often a connection between the frontal sinus and the space under the forehead skin. If you blow your nose, air can be pushed into that space. If you push the skin with your fingers you can hear a typical crackling sound. This is absolutely harmless.
This air escape will disappear in time when the skin is reattached to the bone. To help this process you will be provided with a forehead mask, which you should wear for two weeks. This mask will push the skin against the bone. Also care should be taken not to blow your nose too hard. After two weeks you can take the mask off and carefully try to blow your nose. If still air escapes to the space under the skin, the forehead mask must be used longer.