I've ridden about 1000 km with my Tacx Flux S, but had the same noise issue since the beginning (bought in December 2019).
I opened the two plastic covers (even if still under warranty.. nevertheless the shop is not going to reopen soon with what is going on these days) to look inside. The noise always appears at the same angular position of the wheel which hosts the cassette. At this position the roller wich tightens the belt slightly moves. I don't know if it comes from:
- the belt itself which I believe is slightly deformed.
- the spring lever wich hosts the roller (I did not disassemble it).
I greased the mechanisme where I can, but it did not reduce the sound. Have you already had similar issues? Is it possible to order a spare belt?