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Silicon vibration dampening gaskets wrap completely around the frame of the fan providing a cushioning surface around the entire edge of the fan where it comes into contact with the metal case. These are also more versatile than fan mounts because they will work on any thickness of case fan. The most common size is 25mm, but some are 38mm for extra air pressure. With standard fan mounts, 38mm is too thick and it will not even fit. Since you still use metal screws, you can be guaranteed a tight fit over the silicon fan mount screws.
Pictured: Vantec Fan Vibration Dampening Kit (Comes in 4 sizes, includes screws, 2 silicon gaskets, and 8 silicon gromet o-rings for further dampening).
Vantec also makes a special dampener gasket for power supplies.