I'm with Pat as well. You may simply have an intake gasket problem.
Pull the valve covers
Drain the
oil
Top off the radiator with water
Use a pressure tester to pressurize the cooling system to 14 to 16 lbs.
By looking through the holes and passageways between the head and manifold you should be able to see the 4 corners to the head to manifold.
See if you lose pressure on the tester.
If you do, look in the corners for water leakage.
Also, it is possible if you are using an old manifold some electrolysis may have created a pinhole in the manifold somewhere. I had the same thing happen inside the thermostat area of an old intake.
What type of heads, manifold, and gaskets are you using?
--Mike