Pump runs smoothly on start-up ( cut in at 30 psi out at 50 psi ). Bladder tank at 40 psi. Just last week ( after years of smooth operation ), the supply line from the well to the pressure switch bangs or hammers at cut out. Is there a pressure relief valve in-line that may be failing or failed ?

Is there a pressure relief valve in-line that may be failing or failed
No.
A "hammering" effect on a well pipe sounds more like switch is cutting off and then immediately cutting back on. Sounds like debris is partially blocking port to switch. Remove and clean or replace.

If cut on pressure switch is set to 30 lbs, tank pressure should be set to 28 lbs.