Hmm. This seems to be a regular issue with this bike when the chain is getting old. I had exactly the same problem with mine and it was really bugging me for a long time. I felt my chain was ok, yet it continued to happen. I will guess that you have at least 20,000 kms on your chain or thereabouts ? A replacement chain and sprockets got rid of it for me.
My original chain was ok at that mileage. There was some wear on the sprockets and it was well lubed but there were some tight spots. However I still felt it had some life in it. Eventually my Tech persuaded me to put on a new chain and sprockets and the noise went away.
I tried to rationalise this. I believe it is down to the fact that the NC has a floating front sprocket. That means it is free to move on its splines. When the chain gets tight spots with age, I believe the tight area of the chain causes the front sprocket to rattle momentarily on its splines due to the minute change in chain velocity as its tight spot passes over the teeth of the sprocket. That is all I can think of. If you listen carefully the knocks will coincide with each full revolution of the chain over the front sprocket. Usually when the chain warms up sufficiently, the sound almost disappears.
Anyway, my Tech suggested that the chain was giving me a warning. He suggested that I should heed the warning as otherwise the next communication from the chain might be disastrous. If you have a relatively low mileage chain then I would have to question your maintenance of it.