If it's of any help to others regarding tailgate knocking noises; Mine started doing this a while back, lubed the catches and rubber adjuster stop etc with vaseline , this seemed to cure it for a while then came back, on closer inspection I noticed that the hatch rubber seal seemed a bit on the hard/ less supple side as it were and where the seel meets the tailgate, that had traces/ lines of rubber on it, so cleaned all this off using some label remover, then rubbed some AutoGlym Vinyl and rubber treatment on the hatch rubber seel and on the hatch adjusters, doing all this seems to have stopped it and also I notice now that when opening the tailgate , it opens without having that feel as if the rubber seal was sticking to the tailgate if you see what I mean.
Cheers Pete