Check around the cylinder head for any leaks. Hopefully it's not the head gasket. Also, look at your freeze plugs and the coolant drain plug on the block itself. Should be on the driver's side below the exhaust manifold. Otherwise I'm not sure. At the very worst it could be a crack in the block. Not trying to scare you, just listing the things it could be. Someone else may know another possibility. If you don't find anything that looks for sure like it's the source clean the everything real good with engine cleaner, then look and see if you can see the source of it after driving it for a few days.