I have a 600 foot deep well running a 1 1/2 pump. There is an additional check valve at the 250 foot level. When the pump shuts off, I here a series of clunks, Two or three, each getting softer. It has done this for years. Now it is starting to bug me. I changed the check valve and the pressure switch. No dice. The pump was also changed a couple of years ago. ANY IDEAS ?