Hissing/Whining Sprinkler Valve

I guess Springtime's the time of year for hassling with your sprinkler system. Oh joy.
I've noticed brown spots in my lawn, and after a thorough investigation, it would appear that the water pressure on this particular line is significantly less than on the other lines. There is one additional head on this line than the others, but I don't think that's the problem. The pressure is so low that two of the pop-up sprinklers don't pop. Normally I would guess there's a break in the system somewhere, but on this same line, the sprinkler *valve* is extremely noisy, emitting a hissing, whining sound when the valve is turned on. You can hear the water rushing through the valve assembly like Niagra Falls.
I disassembled the valve head, looking for an obstruction or foreign object of some kind, but saw nothing out of the ordinary. When I reassembled the valve head, the pressure was even worse than before and the noise seemed a bit louder. I'm not sure where to go from here. Any suggestions would be appreciated. :)

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