I'd like to drive a 24 VAC solenoid (an Irritrol series 700 sprinkler valve solenoid) with the secondary voltage from a plugin transformer (a Vitek model VT-24VAC/20).
Checking this on the bench, I separately connect/touch the two solenoid input leads to the two transformer secondary output contacts, the solenoid "clicks" but...
Problem: The solenoid continuously buzzes.
Question: What does the buzz indicate?
(I reviewed Circuit for opening/closing sprinkler valve solenoid?)
The force exerted by the magnetic field from the AC solenoid is 'smoothed' by a shading ring (C in the below diagram, from here):
There is still some force ripple at double mains frequency which can cause sound (and maybe, if it resonates with something loose, a relatively loud sound) from parts moving or changing dimensions slightly in the field.
They can make AC solenoids and relays relatively noise-free, however in the case of a sprinkler solenoid they may not have bothered since it will typically be used out of doors (I think) and any noise will likely be drowned out (so to speak) by the water spray sounds.
If you are repurposing the solenoids for some other application and the noise is an issue you might be able to run the solenoid from DC, however it will have to be a considerably lower voltage (or the coil will burn out) and you might have to experiment a bit.
It's an AC solenoid. They all buzz to an extent - it's unavoidable. Even if the buzzing is inaudible from a distance you can couple an accelerometer to the solenoid body and the accelerometer will pick up the buzz (with said accelerometer being otherwise insensitive to magnetic fields).
Please record the buzzing on a short video and add it to the question. That will help determining whether it's a reasonable amount of buzz or not.
When you drive a solenoid with AC, parts of the device will vibrate in sync with the AC supply due to the continuously varying magnetic field.
