Water hammer can be tricky to figure. There are a few different reasons why this happens. It's impossible to tell over the net. Take a look for yourself. You'll quickly see that even a seasoned veteran might have to rack his brain to solve your problem. The flow control is usually right in the middle of the valve. --
http://www.irrigationtutorials.com/waterhammer.htm
If I can't fix it, it's broken!