back up alarm sounds when first put in reverse

Hello, It's very hard to say until you can test the system to see what is happening to the controls of the system. Back up alarms utilize the sensors to detect things as they near the vehicle. It is common to have one or more of these sensors damaged due to the bumper getting tapped by other vehicles or an object. If a sensor gets damaged, it can malfunction as you are speaking of -- sometimes leading to beeping that continues as if you are right on top of an object when you are not. I would suspect one of your sensors is damage or malfunctioning, and by testing you can find out which one or ones.