We keep hearing a phantom fog horn sound. We think it is coming from the plumbing. We hear it even when no water is running, but at times it seems to happen when the automatic sprinklers go on. It used to be mainly in the morning, but now it happens at night as well. What can this be?

Shut off your sprinkler for a day or two to see if the noise goes away, I think it will. In this case your valve to the system has a vibrating washer. These are rebuildible but I have not done one for quite some time.