The sound a bull makes is called a bellow. Bellowing is a sign of duress and aggression in bulls. Other signs of aggression include kicking, butting, walking the fence and making various body postures.
A person should never turn his back to a bull. If within the flight zone of about 20 feet, the person should move slowly. Swift movement invites a chase. Backing away slowly to safety while facing the bull is the best measure of self-protection. In some instances, a bull showing a threat display will withdraw and turn away if the opponent moves to the distance of 20 feet or more away.