A rubber washer is a ring made of rubber used in mechanical devices. It is used to prevent vibration from spreading from one part to another, reducing the noise levels.
![](http://upload.wikimedia.org/wikipedia/commons/thumb/a/aa/Rubber_washer.jpg/220px-Rubber_washer.jpg)
Typical uses are mounting computer parts, like fans and hard disk drives. By decoupling the motor from the computer case it prevents the resonance chamber effect from amplifying the noise.