The answer to 'why is my supply line foul smell' is that a foul odor coming from your plumbing supply lines is typically caused by bacteria growth or a blockage in the line. This can happen due to stagnant water, mineral buildup, or organic matter getting trapped in the line.
Some common symptoms include a rotten egg or sewage-like smell, discolored water, and potentially reduced water pressure. The solution often involves flushing the line to clear out any blockages or disinfecting the line to kill off bacterial growth. If the issue persists, you may need to replace the supply line entirely.