When we turn our water off from the bathroom upstairs or if the toilet is done running or when the washing machine turns off and goes to the next cycle, there is a loud horn sound after the fact. What would that be?

Sounds like you might need an anti-hammer device installed or check pipes too see if they need support strapping.Also have your local water company check the pressure entering house.Also a worn out rubber washer in a fixture is something to considerate.Peter