2004 X5 PDC issue

For the last 2 weeks.. Everytime I switch the gear into reverse the PDC gives a single long tone. I've done some research on-line and most talk about the their sensors getting wet from the carwash but a day or so later their system returns to normal function. I like to keep my vehicle clean but all car washes as we know use water. Please know prior to the 2 weeks my PDC system operated normally and the pdc button does shut off the tone.

Any help would be greatly appreciated.