Dual flush toilet noise

Just had a dual flush toilet fitted all works OK.
BUT, there is a loud clonk noise as the valve shuts off at the end of the short flush. After the long flush there is hardly any noise.
My sister has a similar toilet but there is no clonking noise at the end of the flush, is this a fault ?, is there any way to stop the noise?
Does anyone know of a site that explains how these dual flush systerns work?? Ant info appreciated.
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