Seagate has long been the silence leader in hard drives, though that leadership position has waned over the last year. I must admit I have a Hitachi and sometimes it emits, for no good reason, a series of beeps. It kind of freaks me out, honestly, even though the drive diagnostic utilities and SMART check out okay. It's been reliable for the last year, but...it still scares me.
You might want to check out those Samsung drives. I know some people would scoff at the idea, but they're one of the few guys that continue to provide a 3 year warranty on their drive. Almost everybody else has shifted over to one year warranties. Though they typically don't perform as quickly as WD or Seagate or Hitachi, they're among the best in sound level and temperature.