The answer to 'supply line foul smell' is that a foul odor coming from a plumbing supply line is often caused by bacteria growth or sewer gas leaks. This can be a sign of a more serious underlying issue that requires prompt attention.
Some common causes include a cracked or damaged supply line, a broken or dried-out P-trap, or a blockage in the drain system allowing sewer gas to back up. The smell may also indicate a problem with the water heater or other appliances connected to the supply line.