: I have a cold water leak somewhere in (or underneath)my slab. There is no evidence, inside or outside, of water; but the hissing is unmistakably a leak. It is slowly getting worse. Is there any way to pinpoint the origin through sound? Toilets have been eliminated as the source. The missing is loudest at the two bathroom sinks. At some cold water pipes, such as the kitchen and laundry room, it can not be heard at all. I have used a stethescope to amplify the sound. The house is 40 years old with copper piping.

Russ,
You are attempting the most difficult of all plumbing related exercises.
Trying to find the elusive slab leak. My preference is to hire someone with a listening device, which is better used by an experienced listener. Not your everyday down at the coffee shop listener, but a pro. Digging unneeded holes in your home should be avoided. an example.

When I was a young pup in the trades, my Journeyman plumber and I were sent out to yet another slab leak in an apartment building our company had plumbed. We guess as good as we could, and then called in the people with the electronic goods. He spotted the leak in the bathroom near the toilet. We proceeded to follow his instructions, removing the toilet, and jack-hammering the concrete slab. At this point, the tenants were concerned that they could no longer live in this construction zone and pleaded for comfort and quiet housing from the General Contractor. The General was very nice and promised them deluxe appointments and a fancy dinner at the expense of my boss.
But you know, we just couldn't find that leak in the bathroom. Now that the bathroom was all ripped up, the expert decided that maybe the leak wasn't there. We proceeded to bust a trench across the apartment, across the dining room and into the living room. Lo and behold, there was the cause of the leak. a nice round hole, made by a pick, with a clear plastic vapor barrier laid over the pipe. The plastic had done a good enough job of sealing that the tenants didn't complain about the noise for eight months after completion of the apartment.
Well, my journeyman and I looked at each other and smiled. It sure didn't look like a bad plumbing joint that someone had left under a slab, but rather something that the General Contractors helpers had done before pouring the slab. Talking it over, we decided that the tenants were going to be talked into McDonalds burgers that night, if the General could talk them into it.

Now did this convince you to get an expert? Hmmmm, Maybe this is more like one of those Complete Idiot entries to win an autographed book from the author.
Oh well, you can't go to work without learning something.