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.