The answer to 'drain trap foul smell' is that a smelly drain trap is usually caused by a buildup of debris, bacteria, or dried-up water in the P-trap. This can allow sewer gases to escape into your home, creating an unpleasant odor. Common symptoms include a strong rotten egg or sewage-like smell coming from the drain.
To fix a smelly drain trap, you can try a few DIY solutions first. Start by pouring a cup of baking soda down the drain, followed by a cup of white vinegar. Let it fizz for a few minutes, then flush the drain with hot water. You can also try using a drain cleaner specifically formulated for eliminating odors. However, if the smell persists, it's best to call a professional plumber to inspect the trap and pipes for any deeper issues.