It is the pressure relief valve.There are several possible causes for it to leak. A defective auto-fill regulator can allow the pressure in the system to creep up and cause it to discharge.Close the supply valve to eliminate that as a possibility. What does the pressure gauge read on the boiler? An expansion tank with a burst bladder or a flooded tank can cause the problem. Tap on the tank, bottom then top. there should be air in the tank. Let us know the results of those and we'll keep going.