Clean the valve and solenoid.
Turn off the water. Unscrew the solenoid from the valve.With the solenoid removed from the valve turn on the water. It is normal to have water spray out of the place the solenoid was (protect yourself from getting wet from the shooting spray). You may notice bits of debris in the water. If no water is coming out there might be something blocked in the passages underneath where the solenoid was positioned on the valve. To perform a thorough cleaning remove the lid from the valve (normally held on with screws) and clean out the interior of the valve. You may want to make a quick sketch to make sure you re-assemble the valve correctly. Next, clean the solenoid as well as the plunger. Make sure you clean well under where the solenoid was postitioned on the valve. The plunger should easily slide into the solenoid. If it does not move easily in and out or doesn't spring back you will need to replace the valve. Now test the valve again using either an electronic repair tool or a valve acitivator. If your solenoid is still making a buzzing noise the problem is a bad coil in your solenoid. Replace your solenoid as well as your valve. You also can simply purchase the same brand and model of your existing valve and just replace the diaphragm and . If replacing a solenoid it is always a great idea to go ahead and aslo replace the valve at the same time. Once your solenoid goes it is only a matter of time until the parts of the valve start to malfunction too.