Hi, I was reading up on that just last week. Here's a cut and paste of what I found.
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THERE IS A LOUD BUZZING SOUND FROM THE SOLENOID WHEN THE VALVES ARE ON
1. The wire size may be too small to allow enough voltage to turn on the adapter.
2. If the solenoid is not getting sufficient voltage from the timer, the solenoid will buzz. You can determine if the timer is the problem by switching the hot wires at the timer (for example switch the wires between stations 1 and 2). If the problem is corrected by switching the wires, and the station that worked fine before the wires were changed is now buzzing, the problem is the timer.
3. this problem can often be solved on models with square solenoids by changing the orientation of the solenoid coil assembly. It can be rotated on the solenoid tube by turning it from side to side. To see if this will correct the problem, turn the valve on manually from the sprinkler timer. With the valve on, reorient the coil assembly until the buzzing sound is very faint or gone.
If I can't fix it, it's broken!