KW Suspension - Ventilation Disc

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I have KW DCC Coilovers fitted, the rear dampers have a plastic token which sits loose at the bottom of the piston apparently called a Ventilation Disc according to the KW spare parts list. I believe this to be vibrating when driving and the source of the noise I am hearing in the cabin.

Before I start messing about with it does anyone know what purpose it serves as I can't find any info online?