Hi
If you have tried the obvious places for the light knocking noise and still cannot find it try this... look at the battery box or fuse box cover under the bonnet, as my covers were on properly but still knocked going over small bumps, try looking at the base of the spring in the suspension, I also has a small broken piece that would make a noise and go when wet and return when dry! thus causing the knocking sound over small bumps and try spraying some silicone spray oil over the moving parts of the suspension (not the breaks) as the road salt can dry out some rubbers/components..all these were problems that I had over the years that took ages to detect despite mechanics looking at them and fixed the problems...