The water solenoid valve feeding our AprilAire furnace mount humidifier cycles very rapidly and noisily. The rapid staccato of the valve rivals that of a machine gun at times! The humidifier works OK, then as the humidity changes the valve starts rapid cycling in response to changes in the sensor output.
The humidifier and water solenoid is relatively new, the sensor controlling the solenoid valve dates back to our old Sears humidifier installed in the early 1980s. The 3-year old AprilAire came with a new controller but installing that would have required cutting a HUGE hole in the return air plenum so I kept the old controller.
Reading AprilAire's installation instructions points to the short distance between the sensor and the humidified air return flex-hose to the furnace plenum as the likely cause of this rapid cycling of the sensor problem. AprilAire has since introduced a new controller requiring only a small hole in the ductwork, so I see two options for solution:
Install the new style controller with an additional transformer and hope the short distance doesn't cause problems again.
Install a 6-inch diameter spin-in flex duct adapter, with air scoop, in the current location in the return air plenum to accept the flex hose from the humidifier. The scoop would be oriented so as to shield the sensor element from the incoming humidified air.
I really don't want to butcher up the return air plenum by relocating the duct or relocating the sensor - if I can avoid it.
Pics would help a lot. Verify weather or not the 24 volts to the valve is short cycling before doing anything. If it is i would move bypass or humidistat. I don't think it would be the valve but you never know until you check it out.