The answer to 'why is my backflow preventer making gurgling noise' is that a gurgling sound from your backflow preventer is usually a sign of a blockage or obstruction in the device. Backflow preventers are designed to stop contaminated water from flowing back into your home's clean water supply, and a gurgling noise indicates that water is having difficulty flowing through the device.
Some common causes of a gurgling backflow preventer include debris or sediment buildup, a malfunctioning check valve, or a problem with the air inlet valve. In some cases, the gurgling may be caused by a problem with the plumbing system upstream or downstream of the backflow preventer. If the gurgling persists, it's important to have a professional plumber inspect the device and diagnose the issue.