Tobi, what can I do next? Do I have to recompile the kernel and change the options?

It seems that the acpi-cpufreq module is compiled into your kernel and that the ondemand-governor is working, which is the wanted behavior.
So there is either a problem with the temperatures or, since there seems to no possibility to control the fan from the OS, the fan control in the BIOS is misconfigured.
I would at first have a look at the temperatures, you can use the programs in the lm_sensors package for that or just use one of the plugins that come with the most desktop environments.
If the temperatures are OK I would look into the BIOS if you can find fan-control functions.

Thank you for your reply.

There are absolutely no fan-options in the BIOS.

I've attached the temperatures. temp1 min./max. is 40 °C, but the value is also constant: 40 °C. If it changed to lower and above 40 °C I would understand the noise... All other values are also in the rated range...

Is this when the system is idling or under heavy load?

Idling. It's very unusual, that the system is under heavy load. This up and down howling seems not to depend on load.

@Tobi: It was the GPU fan. I was sure it was the CPU. I set a fixed fan speed in the nvidia setup program.

Thank you again.

Nice to hear that you figured it out. Please mark this thread as solved using the thread tools at the top of the thread.