Noctua NF-R8 & NF-S12 Fans Review
 

Here we have the Low Noise Compensators, essentially rubber plugs for installing the fans into the case. The concept is that you push it in through the back of your case, align the fan and push it onto the plugs. This works - in theory. Realistically the expanded part of the plug is too wide to fit through either fan's mounting holes completely, meaning you have to slightly shave down at least one side of the plug (or widen the fan hole) if you want to install the it all the way using these. Of course there is a partial install option (just push it as far as it'll go), but that can get wobbly after some time. You can still use the provided case screws, but they won't dampen vibrations as much.

http://www.mikhailtech.com/content/view/218/65/