The fan on my Kasli 2.0 spins smoothly and almost makes no sound before. But recently the fan seems to spin at 100% all the time and makes a lot of noise. Is this a serious problem and how can we fix this?

Usually not a serious problem. If this bothers you and you purchased it from M-Labs, write to the helpdesk.

And this doesn't have to do with the fan speed, but with a manufacturer that continues to supply heatsinks fitted with low-quality fans that break and/or become noisy after a short time, despite a few complaints. We should probably stop using their pre-mounted fans and install something better ourselves (Sunon fan or similar).

13 days later

We also have the same issue with a Kasli 2.0. Right after power on, the fan noise is high-pitched and unstable. After about a minute, it starts sounding like a fan should sound.
As of today the high-pitched noise is gone. I didn't check yet if the fan is still spinning properly.
What would be the expected behaviour (of the Kasli and other cards like Urukul, TTL, ...) if the Kasli is overheating?

The FPGA overtemperature protection kicks in and the Kasli FPGA shuts down.