Hi,
I had the "knock" for a while, checked and re-checked the front suspension; including dismantling everything and testing/inspecting all bushes, brakes, drop links and ball joints.
I think I've found my problem:
The shock strut lower bush bolt bushing was not centred in the bush. Over time, the upper rubber part of the bush had become soft and cracked, permitting the bushing to move and touch (knock on) the upper edges of the bush when there were sudden suspension movements (the weight of the vehicle on the shock strut loads the lower strut bush so that the bushing (bolt) takes the weight, forcing the bushing upwards, via the lower suspension arm, as shock strut/ bush is forced down.
Without removing and inspecting the shock absorber's lower bush, it would was impossible to detect as there was no visible play, noise or damage.
Hope this helps someone. I would add a diagram/photo but I can't find how.