You won't hear a buzzing sound from the solenoid when you power it from three 9-volt batteries in series. The buzzing only occurs when the solenoid is powered from AC, provided by the transformer in the valve controller. However, if you're next to the valve when you apply 27 VDC, you should hear a "click" as the solenoid pulls in. You may want to check the voltage of your batteries under load, to make sure that they're providing 26-27 volts initially and at least 18 volts continuously after the solenoid has been energized. If the batteries are fresh, this shouldn't be a problem. If you have an AC voltmeter handy, check the voltage at the solenoid while it is being operated by the controller. The controller should produce at least 24 VAC "open circuit" and at least 18 VAC continuous under load at the solenoid.