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I have 5 Hunter antisiphon valves in my system. When one of the valves comes on it chatters like crazy for about 15 seconds before it finally opens up completely. This is my highest flow leg on the system. The other 4 valves open and close fine.

What is causing the chatter and more importantly, how do I eliminate it?

Thanks,
Geno

The zone has lots of slope. The line coming from the valve splits and goes both up slope (rise of about 1 foot) and down slope (drop of about 6 feet.

This zone has lots of pop up sprayers and the flow in it is much greater than any of the other zones I have. I was contemplating splitting this zone in two, where one zone goes up and the other zone goes down.