Did you check out the bearing in the CB. Are they sealed or did the seals fall out and have gotten dirt in the bearing and is causing this problem. Or it could be a crankcase bearing thats going out or already done for. Check out the bearing as if there's a sound coming from around that are and it's not the clutch shoes and any of the clutch components then it's gotta be something right. Check those and the engine bearing's as this could also make a screaching sound.

Check for any play int he bearing or a seal that is missing and things like that.