Why is my water softener hammering during recharge?
Water hammer occurs when water travels along long straight plumbing pipes and comes to an abrupt stop. When the water flow stops instantly the forward momentum slams into (most likely) some 90° pipe fitting at the end of the long run.  Others may have different suggestions, but to my knowledge the only way to stop the water hammer is either add pipe elbows to the long straight pipe run, or add a water hammer arrestor near the end of the long straight run.  In any case, it should be addressed because water hammer can cause stress fractures in your plumbing leading to a failure and water damage.