Bob Paine could be right but first check you haven't a pressure valve in the cold in supply near the inlet it's white with knurled knob on top. Turn the knob clockwise to close the valve then open a cold tap turn the valve until you hear a click THEN STOP turning turn off the tap and you should find the pump has stopped. This is common with pressure valves they need re-adjusting occasionally.
Bob Whiting, May 2009