there are four plugs on the pipes, where they cast the water jacket into the pipe, so they could get the casting sand out. these were welded up afterwards with an aluminum plug, and they slowly corrode out over time, eventually leaking. look for what looks like cracking paint around the plugs, usually it is a sign that the plug is leaking water under the paint and it's peeled it back. since you have the tools and the skill, you can replace the aluminum plug on your own for almost free. some people have used jb weld, or drain plugs, but a TIG is definitely the way to go.