Certain 320/321 with a specific model of AMU in the Comms set up will generate spurious selcals if box 2 has an atis frequency set in the standby window. Don't ask me how it's avionics black magic stuff. Just deselect the atis freq on box 2 and it's fine.
This was our case. Is this "deviation from original design objectives" documented anywhere, or is it something you've noticed?