I haven't tuned a FiTech yet, but if it's like a Sniper or a FAST try opening the butterflies to let more air in so the IAC is only being used to fine tune idle, not the primary idle air source.
I did a gen 3 hemi in a 70 Challenger and the IAC would scream at a idle and coming down to a idle. The problem was the IAC was almost closed so I closed the throttle plate so the IAC had to open and the noise stopped. You could still hear it but it was livable. And no more screaming and whistling.
