loud humming when valve is on
I have 6 stations for my backyard, they consist of Irritrol 2700 anti-siphon valves with filter attachments. I unscrewed one of the filter attachments and after I put it back on, this particular valve makes a loud humming noise when the valve is turned on. I called Irritrol and asked one of their support people what could be causing the noise, and he didn't have a clue. He sent me a replacement solenoid, thinking that might be the problem. It was the wrong solenoid though, but then I decided to try switching solenoids with one of the other stations to see if the problem followed the solenoid - now BOTH stations make the loud humming noise! None of the valves that I haven't messed with make this noise :-)
When the valve is on, I can feel quite a vibration in the anti-siphon part of the valve. I think the problem is probably trapped air in the valve, but I don't know how to bleed it out. Any other ideas what might be causing this noise, and how to fix it?
Help! The neighbor's dogs are howling along with my irrigation valves :-)