The answer to 'why is my supply line making gurgling noise' is that the gurgling sound is likely caused by a blockage or obstruction in the supply line. This can happen when sediment, mineral deposits, or other debris accumulates over time and restricts the flow of water through the line, creating air pockets that produce the gurgling sound.
Some other common causes of gurgling supply lines include a problem with the water pressure or a leak in the line. If the gurgling is accompanied by low water pressure or leaks, it's a good indication that the supply line needs to be inspected and potentially replaced or repaired.