1970 Kawasaki G4 Trail Boss 100

Hey yall. I picked up a nice 1970 Kawasaki G4 TR 100 a few weeks ago and I have been going through it doing all the basic things one needs to do after 53 years of neglect (replace all those crusty soft JIS screws, carb tuning, ect...). It runs great, and previous owner told me engine was rebuilt by the guy before him, so sometime in the last 2 years ish i'm guessing. It had quite a bit of rust and crust on the chain and lots of slack when I picked it up. I tightened this up to spec just so I could take some short rides as I get the tuning right and it makes a hard clunking/knocking noise in 2nd gear but only under acceleration. Doesnt do it at idle or any other gear and it isnt tied to engine speed. It seems to be more tied to wheel speed as it doesnt change with revs but gest louder/faster the faster I go in second gear. Gear oil has been changed and was very dirty but didnt seem to have any metal particles in it. Any ideas here before I start ripping into it? Im thinking possible chain and sprockets but for some reason I have this bad feeling about the transmission.