I have what sounds like water running through the pipes in some walls in my house. I have shut off the water at the main valve outside and the sound is still there. I have also shut down the air conditioning in the house thinking the sound may be coming from the compressor. I am out of ideas as to what is causing the sound.

And you can see no moisture? Where is the sound the loudest"? Is it a plumbing wall? Are you in a single family residence , at or condo? Back to you, Tom