I tried almost everything: shorted jack, shorted XLR, emm-6 microphone, output from the other sound card. No difference. I turned off my desktop, my wi-fi router, all LED and CCFL lights. No difference. I think this signal doesn't came from outside. Why do I think so? Because 23kHz peak has always the same level, regardless of the gain settings. I hope links to a few screen-shoots will work. I tried to capture noise level with min-, mid- and max-gain. When the gain is set to the maximum, the peaks are buried in noise. When I was young there were problems with 50Hz and, maybe, local broadcast station. Now we have problem across the whole band .