There are core holes in heads, core hole is how they get the sand out after head is cast in sand mold with molten metal, engine blocks are done the same way.
The plugs they use in these holes are often called "freeze" plugs although it is a misnomer.
The cheap plugs used at the factory will often last the life of the engine, if coolant is changed every 2 years or so, but if not they can rust and start to leak.
If you can get to the plug remove it and replace with rubber expansion plug, they will last longer and are easier to install.
Google: automotive rubber expansion plug images
All auto parts stores sell them
You just need to tap/hammer on one edge of the leaking core plug and other edge will start to spin and come out, it is just pressed in at the factory.
If it should fall into the head and you can't get it out it isn't the end of the world, it won't block coolant flow, obviously best to remove it but if you can't you can't, put new plug in and forget about it.
No, don't use "stop leak" type product unless it is just for temporary use, i.e. to get you home or to the parts store.
The lower intake where it meets the head can also leak, so check that as well
Welcome Visitor! Please take a few seconds and 
