I just happened to play a frequency sweep to see what frequency my headphones could extend to and I noticed a pronounced vibration in the left earpiece which sounds like something in broken in there. I bought these headphones off a person on ebay, what should I do about this?
It may possibly be a normal remnant of regular break-in. My HD600 experienced a little rattle at around 60 hrs of break in, if my memory serves correctly. Give it a little while and see if it goes away. Perhaps play some bass-heavy music to try and help it along.
You can remove the foam pad that protects the driver to see if there's any dirt touching the membrane, and/or open the rear grill (don't know if that's possible – no problem with HD 600); it may very well be just some foreign matter, or a loose vibrating cable.