We have a tough one here folks. We have a persistant knock in our pipes. This is a single knock that you could set your watch by. It occurs every 3-4 min. and it is a single knock. Nothing is on or running or being flushed. We have had 3 different plumbing companys out.We have replaced water softner, toilet guts, pressure tank.We have a well system. We shut off water valve and we still hear it. We turn the water softner to bypass mode, and we still hear it. We have shut off power to the well,it still knocks. This is not a rattle or banging. It is one single knock every 3-4 min. Please help as this is making me go crazy. I dont know what to do and i cant keep paying people to come out. [email protected] Thanks!!