Just 2 months ago, I bought a new Cube Hyde Pro 2020 bicycle with belt drive (Gates, rear cassette out of metal, CDC with 22 teeth) Shimano Nexus 8 internal hub (model SG-C6001-8D). I mostly ride in the city and sometimes for fitness purposes.
Overall I am happy with the bike. It rides quiet and comfortable, but under high torque situations like short steep inclines or fast take-offs I have noticed a single sharp and unpleasant clunking noise from the gear hub. This happens in all the gears. I took a video of it on my daily commute on a short steep incline and gearbox in 1st gear.
What I tried
First I made sure that the tension on the gear shifting cable was correct and that 2 yellow marks on the hub line up when the 4th gear is selected.
Secondly, I checked the belt tension with the Gates frequency measurement app on the phone and it shows ~35HZ tension which is within the range recommended by the manufacturer.
Third, I did some research, and it seems that few other users have noticed such behaviour even on this website. I actually found 2 threads with similar problem but with no clear solution:
I would greatly appreciate any help or input from you. Even if you don't have the solution but have observed the same behaviour, please reach out and maybe we can find the solution together.