External Knocking!

Had my 197 for just over two years now and over the past few months its developed a slight knocking noise at low speeds (below 40mph) on bumpy road.

It seems to be coming from the mid to rear of the underside of the car.

I asked the local garage to investigate it during a service but they couldn’t find anything.
Also scrambled under the car myself for a look but can’t find anything.

Someone help please! It’s driving me nuts!