I've been using my Beyer DT990's for a few months now since I bought them used here on head-fi, and ever since I've had them I've noticed that the left speaker emits a very unpleasant rattling/crackling sound sometimes, especially when playing games, and especially on certain low frequencies. I've tried flipping the headphones around so that the left speaker is over my right ear, and sure enough I hear the annoying sound with my right ear. Also, I tried plugging in my old Grado SR 225's, and the problematic sound doesn't occur.
Strangely, the rattling sound almost never happens when listening to music, mostly just games and movies.
Any thoughts about why this is happening and how/if it can be fixed?