A unique aluminum sandwich variation by enthusiast bluehat, with a large bag of sand below and above. I bet it does silence the drive -- heck, it's practically buried, it ought to be silent!
Unfortunately, not a solution that's practical for everyone. Most people do want their drive in the case & they may have a hard time justifying 20Kg bags of sand in their room (especially to others). The final touch of a tie-dye cloth cover may persuade, though.
Seriously, I am sure bluehat's hard drives are completely silent; mine are not. Here's a challenge: can this idea can be brought into the case?