My house is about 26 years old. In the last few days a "surging/hammering" noise has started, but only when the backyard impulse sprinklers are running. This noise can be heard where the water line enters the house and at the refrigerator (with ice maker). As far as I can tell, nothing has changed. Any ideas what might be causing this?
Thanks for your help.

It is water hammer noise. AS the sprinklers are turning on and off quickly it creates a pulse that travels through the water system of the house. Where ever a water pipe is not secured correctly it moves and causes the noise your hearing. How is the water pressure in the house. Perhaps its set a little high? You can also set your sprinkler system so different zones operate together then you have now. This may stop the noise.