Reversing sensor problem

[bodgit]

The reversing sensors on my cmax have started to make a constant pitch when reverse is selected no matter how far away anything is. I just used them in town in the car park and all was well the usual intermittent beeping as you get close to anything, but 8 miles later reversing into the drive as soon as reverse was selected the noise wont stop. Its the constant pitch when you are about 12 inches away. I've washed the muck off them and put my ear next to each one and can hear a quite fast ticking noise which I believe is normal.
Anyone got any idea's