Weird knocking sound on sprocket

Hello everyone,

I am encountering a weird sound on my bike (see the Video).

I managed to Isolate the sound: it only Happens when the belt is on the sprocket and is pedaling. It already starts with basically no tension on the belt and Gets worse the stronger it is.

There are definitely no bits sticking to the belt and it is also definitely going straight.

Turning the sprocket without belt in the hub doesnt produce that sound.

Maybe someone recognises it and can help me out.
Thanks in advance!