Thank you
kz1000st,
cheapeto,
Maddog, and
bandito2 for the help so far.
When I changed the gear oil, it was full almost to the filler hole (.25 liters). I did have problems with the rear axle nut coming off after 20 or so miles, and I did torque the hell out of it to get it to stay. I don't have access to a torque wrench or the torque specs, so I have most of them tight but not gorilla tight.
After I replaced the gear oil (I used Valvoline CVT Fluid, anyone have experience with it?) the sound was greatly diminished at first, but returned to original volume after a mile. I can't tell if it is still noisy at steady speeds over 30, since engine and wind noise overpower the sound.
No leaks around trans area at all.
Belt is exact size recommended in Linhai's manual.
Shocks are on properly and both rears are at the same load setting.
Rear brake does not drag (I think).
Sometimes it almost sounds like a slipping sound along with the metal-on-metal clank/grind sound. I'm going to make a video and link it here. Hopefully that will give you an idea of what I'm talking about.