My bike makes a similar noise to the one in the video from time to time, mostly when it is struggling to idle too low when cold. My theory has always been that the timing chain rattles around when it is idling too slow, thus the noise - I recall Greg Bender replacing his on his Quota due to a similar sound and it not making any difference. I don't worry about it, and try not to lug the engine. Yes the gearbox is clunky but don't expect smooth once shimmed - I shimmed the trans in my Eldo and it went from ca-chunk to just a clunk. It's pretty easy to do so if I ever pull the Q's gearbox I'll likely do the same.