to stop the bang -- it's water hammer, of course. The first may not be effective -- it often isn't -- but if the flow is low enough you may be able to stop it with a standpipe (I don't suggest something like a Well-X-Trol on effluent). More likely not. The preferred route is a slow closing check valve. There are a variety of these out there, depending on size.
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.
Hoffman Equipped System (all original except boiler), Weil-McClain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch