Three days ago I started noticing my CPU fan loudly winding up and down when idling (with which I mean not running any particular program). I checked the CPU temperature which was and still is fluctuating between 50°C and 60°C. I also have Windows 10 installed as a dual boot where the CPU temperature readings still are about 40°C and the fans are quiet.
I am using Ubuntu 20.04 now for about two months and was observing the temperatures of my GPU and CPU with the gnome-extension Freon from the beginning. The GPU temperature always hovered around 38°C while the CPU was a couple of degrees below that. The GPU temperature is still uneffected so far.
First, I thought the sensor readings might be wrong, which lead me to the sensors-detect command. Running changed nothing though.
I also checked the BIOS where I could also see temperature readings for the CPU and adjust fan curves. The temperature was similarly high around 55°C, though I have read that this can be due to the BIOS just using a lot of CPU load.
Then I checked the CPU load when idling in Ubuntu with top and the built-in System Monitor. All processes in System Monitor had CPU usages of 0 or sometimes 1 which is strange. Top seems to show more sensible data, though even there the total CPU usage does not seem to exceed 20%.
Lastly I adjusted the BIOS fancurve for the CPU fan to the minimum frequency that it allowed. The temperature readings in Ubuntu were not noticeably influenced by this. The readings still fluctuate between 50 and 60°C, still regularly making 10°C jumps up and down.
My Hardware is
CPU: AMD Ryzen 7 3800X 8-Core Processor
Motherboard: ASUS ROG STRIX X470-F GAMING
GPU: Nvidia GeForce RTX 2070 SUPER
What else could I investigate? What could cause this?