It could be coming from either the catalytic converter or the battery. If it's the battery be sure to clean everything well if it has leaked acid. A couple times of rinsing it w/ water and sprinkling baking soda on it will help keep it from eating everything under the hood. It may make a bit of a mess but that can be cleaned up later. Do it the first time when you replace the battery and again in a day or two. If the battery is fine then it must be the converter. It will have to be replaced.