Many toilets leak, and it turns out to be a problem in the mechanism that controls the flushing and filling of the tank. The diaphragm flush valve is one of those places. The trademark symptom of a flush valve that needs attention is a toilet that keeps running, and you must rattle it periodically.
Find the water supply valve, and turn it off. Mark the situation with a sign so that someone doesn't come by and turn it back on during the repair.
Locate the outer cover on the toilet valve. Using a wrench, loosen the cover and remove it. Check the inside of the valve cover for bits of grit or sand that have lodged in the valve assembly.
Remove the inner cover, which sits directly over the diaphragm seal itself. Each step along the way could be the cause. Clean out the space above the diaphragm valve, and lift the valve out.
Examine the valve and invert it. Run the valve under clean tap water to wash away any material that may have found its way into the strainer of the diaphragm valve.
Take off all the parts of the diaphragm valve, and replace them in the correct orientation.
Replace the diaphragm valve, and reinsert the inner cover. Replace the outer cover, and screw it down tight. Turn the water supply line back on, and test flush the toilet.
As with any plumbing repair, take care to check for leaks and to make sure that seals are working properly.