Is there a sail switch in the duct work somewhere?

Ha, had to read up to even know how to respond, weedmeister.

No, no sail switch:

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A Sail Switch activates an electronic air cleaner, a humidifier, or other equipment in response to airflow from the system fan.

Hard to say from afar, but I'd trace back the incoming water line from it's source and make sure it's not laying on top of the ductwork. I've seen them "dance" when the furnace kicks on.

Thanks, tinmantu, will do. I turned the Aprilaire off this morning out of frustration and sure enough, with the gas furnace coming on due to the cold not ANY loud duct noise with turn on and turn off.

Hope those weeks of humidifier use haven't jarred any ducts free from their suspending rods or loosened elbows due to the stress: it's a big system down there in a tight space.

Is it a solenoid valve for the water flow into the humidifier? Perhaps it's hammer. Is it a plastic hose, or copper? If copper, and there's a little slack, a loop in the pipe may help, or, if you're handy with plumbing, a tee with a vertical column for an air chamber might help as well.

Ha! I should have left the pics, alan. Hammer is plausible. Yes, it's a BIG brass solenoid (the prior one was much smaller-somehow I think that this change is related to this incredible turn on of fan and clamping down of the Aprilaire box itself (like being sucked into the supply). Water line is 1/4" copper: I'll look to see if I can loop it. Thanks for your help.

John — OMG, thank you so much! The April air system is directly below our kitchen which we use all the time. You could hear the water flowing loudly through the copper pipe and the solenoid opening and slamming shut constantly. I played around with the water pressure and it solved both problems! I found the right frequency where the water doesn’t make practically any noise and the pressure was low enough that the solenoid doesn’t slam shut anymore. Didn’t cost me a thing and completely solved the problem. I can’t thank you enough for posting this answer to this forum! Some of my sanity just came back.

I had a problem with an erratic banging noise in my Aprilaire humidifier which was the water solenoid switching on and off randomly . It turns out the humidity control on the furnace was set a tad to high which caused an intermittent on and off signal to be sent to the water valve. A simple adjustment of the humidity control to a lesser humidity solved the problem.