I have a 2004 Ford F-150. Recently, I had a back-up sensor system installed. It worked fine for a couple weeks, and then decided to sound every time I would back up even though there was absolutely nothing behind me. I took the truck back to the service center that initially installed it, and they reprogrammed something. Same thing - after a couple weeks of working fine... They have since replaced that unit three times - then replaced it with a different unit - a DT 506 - no brand name. Same situation. The last two times it started doing this was after I had lowered the tail gate. Putting the tailgate up makes no difference. The sound is constant when the truck is in reverse.
Any idea what is going on with this? I have several friends with the same backup units as I initially had and were installed by the same company, and none of them have ever experienced this.
There may be a short in the wiring harness, if the harness has not been replaced along with the unit that may be your source of concern. Is the sensor eye dirty? Does it need to be cleaned or relocated to a less obtrusive location?