What would cause a 3/4" brass ball valve on main (shut off) water line in a house to vibrate (loud noise) when water is running at any faucet in the house? The house is plumbed with ridged copper and is about 5 years old.

The noise is more likely generated by loose stem inside an angle stop under your sinks or at toilet. First, go to every angle stop in your house, CLOSE it and OPEN it again. See if the noise continues. If not, go to main valve and CLOSE it and OPEN it again. See if that helped. If not, go to pressure regulator and turn the adjustment nut 1/4 turn clockwise. Let me know how it went. Back to you. Milo

Not too much to vibrate in a ball valve. What's this valve shutting off? Back to you, Tom