Where abouts under the car?
How old is the car?
I had a knocking from the front under those circumstances, and that turned out to be a fault with the suspension towers on newer 172/Cups. Renault knew about it and sorted it. Took a couple of hours.
That stopped the knocking but I still have a rattle when pulling away with wheelspin or cornering hard. Fred is going to take a look at that Saturday so I might be able to let you know what that is over the weekend. Seeing as my dealer cant see he car for almost 5 weeks.
From what Ive read it could be the lower engine mounts, but Im not at all techincal so what would I know. lol
Could also be the exhaust, but you can check that by reving it when stationary.
Great cars, terrible build quality.
[Edit] the suspension problem was the one mentioned in the post above. They didnt even bother driving the car, they just fixed it.