Humidifer wiring causes sheet metal click noise - is it solinoid?

When my humidifier is hooked up the furnace makes a sheet metal echo click when it turns on and off. Furnace salesman came out and we disconnected the wires at the solinoid and it no longer makes the noise. He suggested it is a bad solinoid. I thought maybe it vibrates when it turns on and off and causes the sound. I took the screws out that hook it to the plastic housing so it doesn't touch anything on the furnace and then hooked the wires back up and I get the noise again. I touched it gently until the furnace power went off but felt no vibration.

I am wondering if the humidifer is wired incorrectly.

Installers took the wire from the humidistat (it has 2 wires (red and white) and ran them first to the solinoid which has 2 wires. (My old furnace had the red wire hooked up to one of the solinoid wires and the white wire hooked up to the other one.) They hooked both Aprilaire solinoid wires to the white wire by splitting the white wire. The red wire was not cut. The wires then go to the inside of the furnace. Red wire is hooked to one terminal and white wire to another.

I know the solinoid is a magnet that opens and closes. I get water running through the humidifier when the blower is on and it stops when the blower is off. Therefore, the solinoid is working properly. I don’t see how it can be causing the noise. Also, when it was hooked up, the salesman turned off the humidistat and I still got the noise indicating that when the humidistat is off there is still an electrical connection—seems odd to me. Could it be incorrect wiring?

There are three things that may move, and any motion could cause some sound. FIrst, you might have a relay that energizes to enable the humidifier. Often, they use a relay attached to the blower motor to enable the humidifier only when the fan is running. Then, when the humidistat decides the humidifier needs to turn on, the humidistat itself could have a click (some are electronic, some have some mechanical component). Third, the solonoid itself will make some sound. The magnet is moving the shaft of the valve which opens the water supply, so there's some mechanical motion and possible sound. The solonoid could also make a humming sound, but most do not have much of that.

So, if it turns on and off both with the fan and the humidistat, it has to be wired okay, or it would have burned up by now. The thing runs on 24vac (at least many of the AprilAIre units do), and at least the control circuit will be 24vac. If it has a motor to turn a wheel, that could be 120vac. Well, if you have a model that has a motor in it, that could be making some sound as well.