I too hear a sequence of loud “click” sounds when bearing down on the pedals going up hill, or getting a fast start on level ground. sort of Whap, Whap, Whap as long as i am pressing hard on the pedals.
The problem comes only when I am really pushing on the pedal, and is easy to reproduce the problem.
I believe the cause is the freewheel is bending slightly when a lot of pressure is applied to the pedals, and so the alignment of the freewheel to the belt is off by a couple mm. That causes the belt to ride up on the the ‘guardrails’ on the freewheel that form the channel in which the belt fits while in contact with the freewheel teeth. So the pedal pressure bends the freewheel a bit, and the belt on the return side rides up on the guardrail, and then falls back into the channel with a loud click.
Can anybody confirm this diagnosis for the loud click, and suggest a solution?