About 2 weeks ago, my water pipes started to produce a humming sound whenever my sprinklers turn on. It sounds like the humming & light vibration are located inside the wall behind the main shut-off valve. The humming only happens when the sprinklers turn on, but not when the faucets and/or showers are on in the house. Sometimes I could hear humming when water is running through my washer, but not all the times. My water pressure is regulated at around 55-60 PSI so I'm not sure this is a water pressure issue. Any help on this matter would be appreciated. Thank you!