I think what you're experiencing may be what is referred to as "Grattle"... or Grado + Rattle. I'm no expert on this but from what you might find in similar threads (punch in "grattle" in search forums) regarding this, it's due to either a crease or wrinkle in the driver things <--- that's technical btw
... OR perhaps it could even be like a hair of yours (or the previous owner's!) coming in contact with or sticking into the driver mesh things.
Probably check out the hair possibility first, apparently more common than you'd think and it's much easier to fix. If it's not... then a suggested remedy to fix the Grattle is to play them pretty loud with bass heavy tracks for about an hour or so. The volume should 'shake' the grattle out, so to speak, and hopefully it'll solve your buzzing problem. It's worked for my SR60s, and for other head-fiers' Grados too.