It is an Astrosyn damper. It is meant to be mounted between the Y axis motor and the motor mount. It reduces the overall sound of the box by isolating the motor from the case...
Have a look at the picture in this section: http://wiki.e3d-online.com/wiki/BigBox_Parts_Preparation#.2814.29_Y-Axis_Motor_Mount The drawing does not show it but the picture...