24 VAC Irrigation Valve Oscillation?
Many irrigation valves use a 24V AC solenoid to operate the valve. The valve is opened or closed by changing the position of a diaphragm or flap which allows or blocks water flow.

During the period of an AC waveform, the voltage of course changes, but the solenoid does not open and close the valve at that frequency because AC solenoids are designed to compensate for the fact that AC voltage crosses zero volts every cycle. It uses a shading coil for this:


  ... a shading coil is designed to obtain minimal pulsing in force. What this means is that the coil splits the pole into two separate parts which causes the flux of these parts to be out of phase. If not for this shading coil, the chattering sound that is commonly associated with AC solenoids would be more noticeable.


Source: Ledex - Comparison of AC vs. DC Solenoids

24 volts is a common choice for irrigation systems because it can utilize smaller gauge wires than say 12 volts, and AC is more suitable for longer distance cable runs, which are common in landscaping applications.