A smelly drain is usually caused by one of three things: a dry P-trap (the curved pipe under the drain that holds water to block sewer gas), biofilm buildup inside the drain pipe, or a venting problem that allows sewer gas to enter the home. For a dry P-trap, simply run water in the drain for 30 seconds. For biofilm, clean the drain with a brush and baking soda/vinegar. If the smell is throughout the house or smells like sewage (not just musty), it may be a sewer venting issue — call a plumber.