Oopsitsbrokenagain

My parking sensors are working very intermittently, more often than not when I go into reverse there's a loud and constant beep for a couple of seconds and then it stops, when it does this, the parking sensors do not work. Sometimes when I go into reverse there is no beep at all and the parking sensors do not work. And very very occasionally I get a small singular beep when I go into reverse and this is the only time the sensors work. Does anyone know what the cause of this might be? I drive a 2007 A4 Avant S-line