Now I have marked one more thing with sound card(s).
Connected sound card output to a deck, headphones to max, deck in source mode, nothing strange. As soon as I connect deck output to sound card input, some strange noise is coming up.
Tried to connect deck output to internal card input instead, again noise but not the same as from external sound card, and at lower level.
Tested to connect MD player to a deck, and deck output to input on the sound card, ne noise. Seems that my sound cards don't like loop mode.