I have a air horn like sound in the house which I believe is coming from the water pipes - I turned off main water valve to the house and noise went away... noise present regardless of water running or shutoff.

Close the stop valves at all toilets and see if that makes a difference.

I closed all the toilet valves (4 in all) and noise continued

OK.

The reason for suggesting you close stop valves on toilets is sometimes you have a toilet fill valve that is leaking and not know it. A loose washer can cause vibration.

Do you have a pressure regulating valve?

I have no clue...

I think I got it... a/c guy clean the outside coil unit and left the water faucet open with the garden hose and flimsy 3 dollar handle as only stop...

Congrats.