Parking Sensor Problems

[d1s]

Hi
Got a 2006 A6 Avant with front and rear parking sensors.
When I press the park assist button, it gives a high pitch beep and the light flashes.
I have listened carefully to the sensors, and one of the rear sensors is not making a clicking noise.
Is there anything I can do before I go straight in and replace it?
Perhaps a way to clean it? Any suggestions?
Thanks

 

[NHN]

Best option is to have it diagnosed with vcds to be 100% sure, otherwise no, swap it out is the next best option.