right...
knocking noise under acceleration....
Assuming it is the same as the one I had then;
It is not by design
It is a fault..
In my cars case, a fault that was present from day one or so the previous owner told me...
He had taken it to Renault several times whilst under warranty and they hadnt got a clue what the problem was....
Simon, as you said, took it to SMC Renault Woking and they knew about the problem and what causes / caused it....
The answer (in my case) was simple;
Loose engine mount -> allowing engine to move back and forth enough to knock the downpipe against the steering rack or something else in that area.....
Its the mount located under the battery that has come loose and starts allowing the engine to flap about.....thus causing the knocking noise....tighten it up and it stops doing it....
Now in my case, the noise returned after a while and when we took the mount off completely this time, we discovered that it was very worn inside and although being fixed at the top by a nut, there was enough rubber missing from the rubber bit to allow a fair bit of movement..
Thre guy made up a nylon insert to replace the worn rubber and hey presto.... noise has abated....
If left long enough, if the mount will fall off.... and er.... well cause problems !!!