I do not hear the warning from the back up sensors (little round pieces on the bottom edge of the rear bumper) as I get close to cars behind me. I do hear the beeping for a few seconds after I put transmission in reverse.

2003 XK8 62,000 mi.

 

Most likely one of your sensors is out. Have someone put the car in reverse while on the brake and put your ear to each sensor. If one is not clicking, that one is the problem. First thing is to check/clean the connector on that sensor.