The banging is caused from the sprinkler valve diaphragm opening and closing. I had the same problem with my sprinklers when I first installed them. Because the diaphragms open so quickly it causes vibration in the piping system. You hear it a lot if you have copper water pipes in your house because the sound reverberates through the pipes. I installed a water hammer arrester on my sprinkler system, but still had the banging. Over time, the banging went away. Your pipes will not blow up, but it is annoying to hear that, especially if they come on early in the morning or late at night. There is not a whole lot you can do about it. It is very common and most people have said that it does diminish over time.