OK I found the reason. Remember to set default format in windows to the same value in which your DAW operate. I found that I had 16bit 44,1kHz set for Windows and 24bit 44,1kHz in my DAW. I changed it to 24bit and problem is solved. Probably switching windows caused bit depth changes which were accompanied by the "pop" sound. But still the sound is present at system power up and shut down. I wonder if in other sound cards it works like this...
