The answer to 'why is my drain trap foul smell' is that a clogged or dried-out drain trap is the most common cause of foul odors coming from drains. The drain trap is the U-shaped pipe under your sink or floor drain that holds a small amount of water to prevent sewer gases from entering your home.
If the trap is dry or blocked with debris, it allows unpleasant sewer smells to come back up through the drain. This can happen if the trap has dried out from lack of use, or if the drain is clogged with hair, food, or other buildup. Fixing a smelly drain trap is usually a straightforward DIY fix, but you may need to call a professional plumber if the issue is more complex.