If you have more pressure than you need for the sprinklers to operate, you might experiment with restricting the flow into the system, to see if it affects the water hammer. Another thing to do, and you should do this anyway when a sprinkler system causes water hammer, is to install a pressure reducing valve in the house line after the point where the lawn sprinkler supply tees off. The closer this point is too the water meter, the less of your household plumbing there is to rattle. Of course, the sprinkler pipe should be solidly braced, so its bumps and rattles don't sound too loud.