My first iMac had screen issues where there were smudges inside the glass casing. I exchanged it for a new one and I've just been using it... Though this one didn't seem as silent as my first one. I mean my brother recently got an iMac also. One difference is, his HD doesn't 'vibrate' as much. If I place my hand on the top of my iMac I can feel it have a soft tremble. On his, I barely feel anything. My HD when it's working... I hear it, I mean it's not loud but it's not something to have to really listen for either. I haven't tested how my brother's iMac sounds though when it's HD is spinning. Other than the HD working at times, I don't really hear anything. But when I shut off my iMac I do notice it suddenly getting quieter. I suppose there is a very low hum from the fans and all that. Just wondering, is this the same for you guys? Or are your iMacs completely silent? I mean if you shut it down, do you notice any difference in sound?