That sound does not exactly follow rpm. When revved, the sound goes up, but when you let off the throttle, the sound continues even as the sound from the exhaust drops, IOW, the sound lags behind the sound of the exhaust. IMO.
The sound reminds me of something being loose on a shaft and wobbling around. It at the same time sounds as if something was being skipped on like a playing card in the spokes of a bicycle. Another example is teeth of something impacting or being impacted(touched) by something else. Definitely not at engine rpm. I would look at the assembly of the gears on the of the crankshaft. OI think one goes to balance shaft, to the spin filter, there is an idler that is trapped by the clutch basket. It may be possible to put an idler on reversed, in for out, and may have interference.
I do not have a good mental picture, but I would take off the side cover, pull the spark plug, and rotate things manually looking for wobbly things and things that touched other things. I did post on site about doing a disassembly/re-assembly of an SG/CG250. I should look at the pictures.
Here
http://chinariders.net/showthread.php?t=27837
The gear ratios point to the plastic gear here
http://chinariders.net/attachment.ph...1&d=1608303385
It is undergeared from crankshaft rpm, the balance shaft is at crank rpm, the clutch basket is way under engine rpm... my takie is something clanking around behind the plastic. I think the gear runs the oil pump. Could be something behind the gear. If the plastic gear got pressured by a lever as you removed the clutch basket, it could be running out of true, wobbling a bit and clonking into other bits or even the oil pump behind. It's been almost two years so this picture is my memory.
I did have problems removing the three Philips on the spin filter cover, but finally got the sticky one out using a hammer & chisel, replaced with 3 allen head screws. When I took things apart, I jammed something between the gear teeth on the crankshaft gear. Things were TIGHT, and I feared damaging something but it apparently worked out.
tom
Looking a bit further down the post...
looked at this one:
http://chinariders.net/attachment.ph...1&d=1608303830
the spin filter has a spring on it....you did put the spring on...?? Don't know if the noise ratio is right. If the spring was off, the spinnyfilter could move around on the shaft.
Actually, no it cannot move like that unless the castellated nut is not tight. Seems the spin filter had to be wiggled around on the shaft when installed from memory. Could the nut be tightened for the filter, but the filter not seated when the nut was tightened, allowing it to move on the shaft, spin, and keep spinning as the rpms drop off(as I think the noise does... in the vid, one case the engine revs drop, and the noise does not for a short while)
tom