One more suggestion has come in from an owner, and I probably should have thought of it earlier. Another possible cause could be that the valve just didn't close all the way due sediment or loose gravel in the valve. Sometimes it might be solved by opening the valve slightly, then closing it again. The opening can dislodge whatever might be lodged in there, and when you close it again, the problem is solved. Probably not in all cases, but maybe in some.