I have a 1997 502 MPI that has developed a loud whistle out of the IAC tube behind the flame arrestor but before the throttle plate. As soon as it goes beyond idle it goes away. This is a audible whistle or squeal rather than the normal hiss. IAC seems to operate fine and idles just fine. Anybody have or had this issue?? Engine has just over 300 hours of run time since new.

The IAC passage can get full of gunk that in turn could cause the IAC to open a little more to control idle. I’d try cleaning the throttle body and IAC passage.

See if throttle body gasket is sucked in.

my 454 mag had a similar noise and was surging, cleaned the throttle body and new iac and it went away