Is the crackling most obvious with low bass notes? There could be a hair bouncing around on the driver. More unfortunate scenario might be a loose voice coil. If the voice coil is loose you might be able to see it...If that is the case return the headphones if there was warranty.
I'm not completely sure about that. I searched for some low bass songs on youtube and i could hear it on them. There is something strange happening though, this crackling seems to be coming and going. On those low bass songs i could hear the noise first, a little while later when i listened to the same song i couldn't hear the noise. I have tried 3 different sources (devices) and this noise is sometimes there and sometimes it isn't. Not really sure whats happening to be honest.