The answer to 'shut-off valve foul smell' is that a foul odor coming from a shut-off valve is usually caused by a buildup of debris, bacteria, or stagnant water. This can happen when the valve is not used regularly or if there is a problem with the plumbing system.
Some common symptoms include a rotten egg or sewage-like smell, discolored water, and potential leaks around the valve. To fix the issue, you'll need to shut off the water supply, disassemble the valve, and thoroughly clean and sanitize all the components. If the problem persists or you're not comfortable doing this yourself, it's best to call a professional plumber to diagnose and repair the issue.