The answer to 'why is my drain trap foul smell' is that a smelly drain trap is often caused by a dried-out water seal or a blockage in the trap. The water seal in the drain trap is what prevents sewer gases from entering your home, and if it dries out, those unpleasant odors can come back up through the drain. A blockage in the trap can also cause water to stagnate, leading to foul smells.
Some common symptoms of a smelly drain trap include a persistent bad odor coming from the drain, gurgling sounds, or slow drainage. To fix the issue, you can try pouring a cup of water down the drain to re-establish the water seal, or use a plunger or drain snake to clear any blockages. If the problem persists, you may need to disassemble the trap and clean it out thoroughly.