Lots of ball-valves drip continuously after a few years service, usually a bit of grit on the washer. As long as the water level is set low enough there's no cause for immediate concern. Even if the water level rises, all that will happen is that the drips will then come from the overflow to the outside. As you use water from the tank (from your taps and toilet cisterns) this is unlikely to occur, unless things worsen considerably. As NazNomad suggests, get someone to get up there and have a look-see - doesn't need to be a plumber at this stage, just someone to re-assure you it is a dripping ball-valve, which is not a real disaster, just might keep you awake!