I have a pair of Grado PS500's and I've only noticed this on certain experimental/noise tracks. I don't know too much about headphones but I assume it's the driver vibrating too much and hitting the sides of whatever it's inside of? Will this get better with 'burn-in'? I only detect it in the left headphone, but I don't know if it's the tracks though or not yet. I bought them used too, is this a problem Grado could address? Could I send them some sample tracks to play the headphone with, if I find out that it's isolated to only one side? Is there a name for this? When I play a similar track on my Alessandro MS-1s the bass sounds like crap and the vibration is super noticeable. Thanks everyone for reading.
 
EDIT: I just confirmed, if I rap on my left headphone enough the sound will go away. Now, with that piece of evidence would Grado address this?
 
EDIT EDIT: I sent Grado an email asking what this was and if it can be fixed. I wish I had a better vocabulary to describe this, if anyone here knows what I'm talking about, that'd be really helpful. I sound rather stupid when talking about sound reproduction. Thanks again all.