I have lived in my house for 30 years. About 2 months ago I started noticing a running water sound in house. I checked all outlets and everything off and no leaks. I checked my water meter and it is the kind with the small black triangle that is very sensitive. It was no moving. My water bill has not changed, and no water in house or outside around house or any other signs of leaking. I called my water dept and they have not changed pressure and my water meter is good. The sound of running water only goes away when I turn all water off in house at main cutoff. My house is one story, on a slab, hot water heater is old (22 yrs) in attached garage. I have also turned off each sink and toilet one at a time to see if there was a leak in that area. NO change in sound. The sound was the loudest in my shower stall. I tore out wall behind shower to see all plumbing, because it sounded like it was running in wall. Everything dry. I am stumped--any thought? Thank you.