Yes, stop using the toilet. Just kidding. Your question actually contains the answer. Almost! No, you don't have to replace the entire valve. All you need to do is to replace the gasket within it. It truly is a 50-cent repair. Turn off the water to the toilet and flush it. That will empty the tank. Next, remove the shut-off-valve cover. How you do this will depend on the brand, however, most have four screws on top. Carefully remove the screw ? and then the top, locate and remove the gasket and use disassembly as a guide for replacement and reassembly. By the way, it is almost always easier to replace the entire unit. Oh, the humming. That's the ballcock assembly telling you it has a gasket that is almost completely worn out! It's true! And, good luck!