It's sound sensors, Mike.
The dealer didn't know. They just said it was working. But it's not loud enough. Front ones are OK
It's been to the local dealer but I also got information form the Mercedes Benz Tech Centre in Holland. They said that both the frequency and volume could be altered.
They were altered but it's only just audible to my wife and I. Nowhere near as loud as my friend's Skoda and another's BMW.
I'm not sure if anybody really knows what they're doing. Thanks for your help anyway, Mike.