Try pressing the affected headphones against the sides of your head firmly with your hands and see if you still get the same distortion. When I heard it, this ``solved'' the problem, which would indicate to me that it's a problem with the body of the headphone vibrating on its own, induced by the sound that's supposed to be happening. But damping the headphones with your hands should ``solve'' that vibration. If that works, you know that the problem is with the headphone body, as opposed to the membrane or cone emitting sounds that it's not supposed to, since it's still able to vibrate freely.