I also slow drive in 10.5T,
I had set 20 degree only but also have this noise and not smooth,
Is ...
maybe you try other Ching's suggestion
1. change the wheel to test first
2. add belt top to limit the belt
3. double check the direction of the ring (which for adjust the tension of belt) both side to ensure both sides are set to same location
4. check the direction of the "Big" gear, "with screws head facing to the rear belt"
I'm newbie only.. so just to discuss, if i'm wrong dont kick me too hard..