The answer to 'shut-off valve foul smell' is that a foul odor coming from a shut-off valve is often caused by a buildup of bacteria, mold, or debris in the valve or the surrounding pipes. This can happen when the valve is not used regularly or if there is a leak that allows water to stagnate.
Some common symptoms include a rotten egg or sewage-like smell, discoloration around the valve, and potential leaks or water damage. To address this issue, you may be able to DIY by shutting off the water, removing the valve, and cleaning it thoroughly with a disinfectant. However, if the problem persists or you're not comfortable working with plumbing, it's best to call a professional plumber to diagnose and fix the issue.