I've had a system in for about a year now, and all of a sudden it started making like a howling noise, and its not just the sound of the water flowing through the pipes. The vacuum breaker is a 1" Watts with all one inch pipe and valves. Zones 1 and 2 are 1800's and 3-6 are i20's. Sometimes for some reason zone 2 will not make the noise but all the other zones always makes noise. I went into the basement and isolated the line coming from the main to make sure it wasn't vibrating againts something and that is not the problem.