Try it from another source, like your computer, with different songs. Or from a CD player. Or from an AV receiver. Use different musics from CD source if possible. See if the crackle persists. In other words, rule out the iPod.
It only does it on some songs (kind of alot of songs tho). Is it probably just the music files? On my computer it hasn't happened, but I'm not sure if that's just because I've been playing songs that don't cause it.
Possibly. Try and see which songs do it, and what the differences are between songs that do it and do not. For example, is it only during bassy music, or is it during quiet acoustic, etc. Figure out if it's a series of tones that happen to rattle. Or if it's just random. In which case, take the song you have that definitely rattles and get that same song again from another source (youtube, pandora, amazon, etc, some where just get another of the same song) and see if it rattles as well. Rule out your own audio as being poorly compressed or simply poorly mastered. The D2000 is a highly resolving headphone, so if there's imperfection and compression or poor mastering, you're going to hear it.
That's a fake vinyl sound. Likely thrown in to give the song a retro effect. You probably havn't heard things like that until putting the D2000s on, because they're way more resolving and revealing than whatever else you were using.
Oh cool. The album that song was from was the first thing I listened to on them and some of the other songs on it had the same effect. I've heard it at the beginning of one other song since then and that's it.