A diagnostic check should point you in the direction of the fault(s).Testing the system manually can be done but is very time consuming.The main control unit sits in the boot,on a bracket above the battery.If you have front sensors fitted,then a fault here will stop the whole system operating.hth

Obvious possibility, which produces these symptoms.

One or more of the actual sensors may have become dislodged inside the bumper. The sensor then "sees" the inside of the bumper and reports an obstruction a couple of centimeters away. Voila, continuous tone.

A mate had this after his wife reversed his Skoda Superb into a post. Nothing obvious from outside (apart from a scrape on the bumper) but continuous proximity tone on engaging reverse. I found the loose sensor and glued it back in place, fixing the problem.