So it probably is a core plug leaking. The core plugs ( people incorrectly call them freeze plugs ) were there so that the sand cores can be removed once casting is complete. A metal plus is pressed in and evidentially they rust and leak. There are "temporary" rubber plugs you can use, but best is to get a new metal plug back in. The hole needs to be cleaned, even sanded a bit. Then some sealer applied to the plug and hammered in using something like a large socket to spread the force around the out edge. Once i tis started, you can more easily tap it around the circumference until it is flush. Don't go too far :)
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