The answer to 'drain trap foul smell' is a clogged or dried-out drain trap. The drain trap is a U-shaped pipe under sinks, tubs, and floor drains that holds water to prevent sewer gases from entering your home. If the trap dries out or becomes clogged, it can allow unpleasant odors to come up through the drain.
Some common causes of a foul-smelling drain trap include infrequent use of the fixture, a dried-out trap, debris buildup, or a broken or damaged trap. Symptoms include a rotten egg or sewage-like smell coming from the drain. To fix this, you can try pouring water down the drain to refill the trap, use a plunger or drain snake to clear any clogs, or replace the drain trap if it's damaged.