I picked these up second hand today and tested them while picking them up from the seller. Nothing seemed out of the ordinary so I made the deal and brought them home, while listening to them at home, I heard a rattle from the left earcup at certain frequencies. I don't think the driver has blown, but I don't know where the sound comes from. It seems to completely disappear when I press on the grill of the left earcup, but this is only a crutch. Anyone have any idea what could be wrong?