Couldn't find anything on the web on this. Our heating system is a hot water baseboard system. The boiler feeds into an air scoop, then a Taco bypass zone valve. The "bypass" outlet goes to a manifold with two Taco zone valves, one for each heating zone. The "controlled" outlet goes to an indirect fired water heater. The piping has been in place for 26 years, the boiler and water heater were replaced about four years ago. Starting with this heating season, the system has been making a "singing" sound that can be heard throughout the house. After somewhat extensive investigation, I've discovered that the noise only occurs when both zone valves to the heating loops are open. Additionally, I believe I've narrowed the source of the noise to the bypass zone valve. What I can't understand is why it's just starting after 26 years and why only when both heating zones are calling for heat. Any ideas?