Interesting you mention this,as my 200 has started doing it. Mine does it over uneven road surfaces; I thought it was the exhaust knocking so took it to Renault. Mech had a look and said exhaust was fine, he showed me what he thought it was. He had the rear diffuser unbolted, and there was a metal heat shield on top of it, he hit the diffuser and the heat shield would bounce / tap / knock the plastic diffuser. He said it was nothing to worry about. I was pleased and drove off. Later I thought - it wasn't making this noise when new, so why now? I think he had seen the same problem before because he described to me exacty the circumstances in which it happens. I hope there is nothing to worry about, I hope Jase is right and it's the boot lock. Can you give any more info lild?