The answer to 'supply line making gurgling noise' is that a gurgling sound in a supply line is usually caused by a partial blockage or obstruction in the line. This can happen when sediment, debris, or mineral buildup accumulates and restricts the normal water flow. The gurgling noise is the sound of air being forced through the restricted area.
Some common causes include a clogged aerator on a faucet, a partially closed shut-off valve, or a problem with the water heater. In some cases, the gurgling may indicate a more serious issue like a leaky pipe or plumbing vent problem. It's important to investigate the source of the gurgling to prevent further problems.