Reversing Sensors

[nwsavvy]

Hi folks. Can anyone throw any light on what has started on my mk3 2006 mondeo ghia. When putting it in reverse it beeps then there is a longer lower toned beep then 5 or 6 short beeps. Is this an electrical check or something? I don't remember this happening before the last couple of weeks since the last frost and heavy rain. The sensors seem to work ok after the initial switch on and if I walk in front of them they work ok and chirp as they should.

Can anyone throw any light on this?

Many thanks

Barrie