After who knows how long, maybe a year or something similar, I power cycled my server and disconnected it completely from power as I arranged my rack again. After booting the fans didn't slow down but kept spinning around 14krpm, and bmc reported their speed value as normal with that rpm. Well, boots, power cycles nor nothing felt like helping.
I updated the bios (is there a newer one, I updated to 3A14 I think even though bmc reports 3A12.02?), and that fixed this in my case. Now bmc reports normal fan speeds with approx 4000 rpm speeds, and the server is quite silent.
I have maybe 8 spinning disks and power consumption is maybe 130 watts perhaps?
Overall, I've been quite content and satisfied with these machines. I'd like the new firmware to be available, but old ones work, too.