The answer to 'why is my faucet foul smell' is that a foul odor coming from your faucet is usually caused by bacteria or mineral buildup in the plumbing system. This can happen when water sits stagnant in the pipes, allowing bacteria to grow and release unpleasant smells.
Some common causes include a lack of use (like in a guest bathroom), a clogged aerator, or mineral deposits in the faucet. The smell may also be coming from the drain rather than the faucet itself. In any case, it's important to address the issue promptly to prevent the problem from getting worse.