As a quick fix.. you could change out the elbow above the checkvalve for a Tee, and put an arrestor above it. if you really want a quick fix you could even just drill and fill it in. Any building supply store generally has them, they're used for basement faucets, toilets, washing machines, stuff like that. They come brass or plastic, washing machine ones are generally plastic, and only run you about $20-$30 (yeah a lot for a chuck of plastic with an air cushion behind it). Also noticed the double elbows for what looks like power cord access, this could be the main problem of the water hammer, if you take that out while fixing your sump just throw a flexi pipe for the corner and you might dodge the problem completely. Anyways, good luck with the sump
