The answer to 'supply line foul smell' is that a foul odor coming from a plumbing supply line is typically caused by a bacterial buildup or mold growth within the line. This can happen due to stagnant water, poor water quality, or a lack of regular flushing or cleaning.
Some common symptoms include a strong, unpleasant smell coming from the faucet or fixture connected to the supply line. In some cases, you may also notice discoloration or slime buildup in the line. The smell is often described as rotten eggs, sewage, or mildew.