The answer to 'backflow preventer making noise' is that a backflow preventer making noise is usually a sign of a problem that needs to be addressed. Backflow preventers are designed to prevent contaminated water from flowing back into the clean water supply, but if they are not functioning properly, they can make a variety of noises, including whistling, gurgling, or humming.
Common causes of a noisy backflow preventer include a buildup of debris or mineral deposits, a worn or damaged internal component, or high water pressure. If the noise is accompanied by leaks or other issues, it's important to have the backflow preventer inspected and serviced by a professional plumber as soon as possible to prevent potential water contamination or other problems.