The answer to 'why is my supply line foul smell' is that a foul odor coming from your plumbing supply lines is often caused by bacteria growth or a blockage in the line. This can happen when water sits stagnant in the pipes, allowing bacteria to thrive and produce unpleasant smells. Common causes include a clogged line, a leak, or a problem with the water heater.
You may notice the smell coming from a specific fixture, like a sink or washing machine, or it may be coming from multiple locations. The odor is typically described as rotten eggs, sewage, or mildew. Addressing the root cause of the smell is important to prevent further issues.