Hi psilunary,

If you are sure the water is at the valve, the plunger retracts upward in the solenoid head, the manual release knob on the valve body doesn't make it turn on, unscrewing the solenoid doesn't make the water flow then the only thing left(except for a damaged diaphragm) is the small exit orifice from the solenoid chamber is clogged. Without enlarging the hole see if you can stick a thin metal wire through it. Otherwise try a new valve. The hum is probably the diaphragm. The diaphragm could be damaged and too much water is getting into the upper chamber overriding the solenoid release system. replace the valve and save the frustration of searching for the actual cause of the problem. JMHO Aloha