Actually, I was at more threads and I stumbled upon the one where markl first posted his famous markl mod for the D5000.
As you can see, directly across from where the headphone cables exits to the other side, there is indeed a hole punched out there too, exactly matching what I've got, so it isn't a defect then. I was playing around with it more and I fluffed the cotton-like stuff that is attached to the inside of the cup, and then on reassembly (with that cotton in the hole removed) my problem actually disappeared by about 98%! If I press my left cup to my ear very hard while listening to music, the rattle completely disappears. This is making me think this has to do with sound pressure. If that is the case, it would explain why covering up that hole, raising the cotton-like stuff, and pressing the driver to my ear fixes the problem.
If anyone has a D2000 and would be willing to disassemble it for me to do a little test on, I would greatly appreciate it. I need to find out if it is indeed my driver, or something else that is causing the issue. If you could, take off the driver from the headband and the cup, so it is at this step:
And then play the most bass heavy song you can find (
or use this one, this is a clip from one of my most problematic songs) at a moderately loud to loud volume (as loud as you're comfortable with). Then let me know if there is any sort of distortion whatsoever, no matter how small. Thank you!!!!
EDIT: I'm not sure why it never occurred to me to just open up the other side. Anyway, the other driver doesn't exhibit any problems whatsoever, so I'm completely sure it's the driver in the left ear. I'm sure my previous owner never noticed it as it's a problem that doesn't present itself in all recording (only quite bass heavy ones) and also only in certain parts of said recordings (such as when there aren't any other instruments playing; if any high note plays during the problematic section, the problem goes away entirely, even though the bass hasn't diluted or stopped). It could also be that my definition of a comfortable volume level is higher than normal, as both of my parents immediately turn down the volume when listening to any of my headphones, and at the levels they find comfortable, I can't discern a problem at all. I think I may just sell these, as someone else is likely to not have a problem with them at all. I do use a little EQ to increase the bass a bit while removing the edge on the highs, and if I remove it, the problem completely goes away (no the EQ isn't a problem, else swapping the channels, both physically and through software, would solve the problem).