Thanks for the replies. To answer some questions: yes it does whine due to the load of the cpu. The system is the same, i tried with more cpus. All of them are working great except the 3.4GHz one. i really don't understand, never seen such a thing. i'm using WinXP and Debian in dual boot. i benchmarked the processors with multiple programs on both OSs. 3.4Ghz performs worse than P4 2.8Ghz without HT enabled. The heatsink is okay (Cooler-Master Hyper 6 - really big one). Max temperature with any cpus at load is under 45°C. And also not the fans making the noise (two 8cm noctua). Motherboard supports cpu. Only thing i can think of that somehow there is a problem with voltages around the cpu.
I'm pretty sure you're hearing frequency interference of some sort. That CPU runs at 3.4GHz though you cannot hear 3.4Ghz it's possible that the CPU's clock signal is malfunctioning in a way that for a very short period of time that it is running at 3.4GHz it drops down to 20 Hz to 20 kHz for a very short amount of time making you able to actually hear the CPU.