This distortion is caused by the difference in air pressure from inside to outside the diaphragm. When you move your head the air volume inside the earpiece changes suh that it no longer matches the atmosphere and thus the diaphragm is momentarily forced to move a great deal, causing the distortion sounds.
I found this with the Omega 2 (!!) and was utterly dissappointed and downright suprised. I mean, you dont expect to have to stay rigid when listening with a $6000 system!
Like you i found that if i mived my head, sharply or even slowly sometimes, or lay on my bed, of touched the headphones, i would get the papery suction kind of sound, which i think is what you are describing. it's not actually a harmonic distorion of the music is it, more like a noice right, irrespective of the music playing. Is this right for what you experience?