Apache:That hole is part of the heated air supply for the automatic choke. There is also a hole on the other side of the manifold unless the automatic choke heat tube is still attached. There is a tube inside the manifold extending from hole to hole. What has happened is that the tube is rotted/rusted out, and exhaust is entering the tube. If the choke is still connected to the manifold on the right side, it is also getting exhaust gasses which will corrode the internal components of the choke, rendering it inoperable. If you are not using that choke arrangement, you can plug the hole where the exhaust is leaking out. If you are still using the original choke setup you will need to remove the manifold to replace the heat tube, of get another source of heat for the choke and plug the holes on both sides.