It smells sour because it is acidic from milk products.
The buttermilk will likely smell sour, although the more accurate way to determine if you’re smelling a natural or unhealthy fermentation is sulfur undertones. Healthy fermentation of butter churned from raw milk produces a fruity aroma of butter and cream with a hint of cow manure, while spoilage brings about an unpleasant odor usually described as moldy cheese.