Since air is trapped between your head and the diaphragm it creates a pressure zone, and as the headphones shift with movement you may hear a slight crinkling sound from the diaphragm. This is a normal occurrence with planar magnetic drivers and is nothing to be concerned about, it's the sound of the thin diaphragm moving back and forth with the changes in air pressure.
Sometimes as the driver film stretches slightly with age, the sound can become more apparent, and sometimes it may also lessen again. This is mostly influenced by factors in your environment (such as temperature and humidity), and we don't have much control over whether or not this occurs.
One important note is that we strongly recommend exercising certain cautions when handling planar magnetic headphones: trapped air can create high pressure and sudden pressure changes could damage the diaphragms, and this is not covered by warranty. The same is true when you put the headphones on or take them off-- it's not a good idea to press them hard and fast against the head, so we recommend slow and steady movement to allow the air pressure to stabilize.