Why does my Aprilaire 560 make clicking noises before starting?
If the 550 is like the 560, they use the furnace's blower and don't have a built-in fan.  In a typical setup (at least for the 550), when the furnace is generating heat, the furnace supplies voltage to the humidifier, which may activate if the relative humidity is lower than the setting on your humidistat.

You should be able to feel the solenoid click when it activates.  You can test this, with the furnace producing heat, by turning the humidistat up and down.  If you get the click, but no water comes out in your 560, and you're sure your water valve is on, it might be a clogged emitter.  They have a tiny pinhole that slows down city water pressure.  Mine is in the water line between the solenoid and the Aprilaire unit.  You can buy new or some people clean them out with a pin.