High pitched "squeal" coming from the pipe where the water enters the house when the water is being run anywhere in the house. Very annoying. The house is about 20 yrs old. Copper plumbing. I have been here about 8 years and it has not always been this way, although I do not recall when it started. I already tried adjusting the main water shut-off valve. No luck.

What do you mean by adjusting the water S/O valve?

Get a water pressure test gauge see the image below,check the water pressure, and let us know, it sounds like you have problem with you PRV, Thanks.

John

By "adjust the valve" I simply mean that I have tried it with the valve partially closed, all the way open, etc.

Check the water pressure and let us know, Thanks.

John

Do you have a PRV,(pressure reducing valve see the image) installed. Answer Johns question. What's the PSI entering your house Back to you, Tom