Multiple check valves on sump pump

[RLdawson]

A recently install Zoeller 508/98 sump pump emits a series of loud "bangs" after every discharge cycle. The plumbing contractor says there's nothing to be done about this and to just "live with it". But the noise is intolerable.
I noted that he had installed a Zoeller Silent check valve in the discharge line. As there is a built-in check valve in the Zoeller pump, is this really necessary and could it be causing the banging problem (water hammer?)?

Thanks!