The answer to 'why is my supply line making gurgling noise' is that a gurgling sound in your plumbing supply line is often caused by a blockage or obstruction in the line. This can be due to a buildup of mineral deposits, debris, or even a kink in the supply line. The gurgling sound is caused by air bubbles being forced through the restricted area, creating a gurgling or bubbling noise.
Some common causes of a gurgling supply line 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 also be caused by a problem with the main water supply line to your home. If the gurgling is accompanied by low water pressure or other plumbing issues, it's best to call a professional plumber to diagnose and fix the problem.