Dell Optiplex 755 MT Ubuntu Linux 14 14.04 noise on audio output / noisy sound card simple solution

I would like to share my experiences how to fix noisy audio output on Dell Optiplex 755 under linux. I did try changing power supply, better video card, adding separate sound card (front panel audio output doesn't work then) updating drivers and even changing the whole computer. I was first sure that the reason for the noise must be old capacitors somewhere in the system board or in the power supply. It took me months to find the solution.

The symptoms are: static noise on audio even if there should be no sound at all. The noise doesn't depend on the audio volume. 

The solution: open terminal, start alsamixer. HDA Intel Analog Devices AD1984 should open. Use right cursor key to scroll the screen until Beep audio level adjust is selected. Lower the volume of Beep down. This should fix the noisy output.  I set the Beep volume to 6%.

Apparently Beep sound output produces low level noise even if no beep sound is produced. If Beep volume is adjusted to high in alsamixer, this noise is amplified and added to the audio mix.