Is there such a thing as a generator breather? Most likely the main engine centrifugal breather. This seperates any air from the engine oil before returning to the oil tank. At low engine speeds the breather (which is driven from the accessory gearbox) does not turn at a high enough speed to completely seperate the oil from the air, hence the air carries a fine oily smoke which is dumped overboard through this vent. The 737-200 engines used to do it alot as does the v2500 on the Airbuses. All in all quite normal and nothing to worry about at all.