It sounds as though the water pump may be cycling. Cycling is when the pump cycles from off to on or on to off. The pressure of the pump cycles the pump off when it has reached the maximum pressure that builds up in the output lines, for example when shutting a faucet off. Rapid cycling happens when the flow of water is slowed down by a restricted riser or elbows, which cause the pump to run harder and run at the boarder of shutting off causing the pump to cycle rapidly. Click to enlarge the photo for more on cycling and what it may be caused by.