Assuming you're referring to the exhaust, it could be a number of things, but most of the major problems have clear symptoms. For example, dense, white exhaust smoke that takes a while to clear is evidence of coolant in the engine oil. Exhaust that comes out blue and smells like oil or fuel is evidence of oil/fuel dilution or excessive oil consumption. If the vapours disappear quickly or decrease significantly after startup, it may just be condensation in the exhaust that is evaporating into steam when the hot exhaust gases pass over it.
If ambient temps are starting to drop where you live, this would make the exhaust vapours more visible as well.