I bought this 250 a few weeks ago and it's had a sort of slight knock to it. I don't think it's anything wrong with it, it's probably just normal. At least that's what the mechanic said, "all the Hondas do that." I'll link a video of it starting and running. Bear in mind the knock sounds worse in the video and it's only at low RPMs.
https://youtu.be/CVCdqR10Qmk