You’d need to talk to AJA if you’re sure the artifact is coming in as a result of a-to-d conversion via the AJA.
The card should be totally isolated, audio wise, from the power supply but it might be picking up RF from any source in your shop. Other sources of audio crosstalk can come from improperly shielded monitors, display cards, bad FW card, Airport card, nearby Blackberry.
First try going directly from the deck to the card bypassing the mixer not because the mixer might be bad but you want to eliminate some cables.
bogiesan