Figured it out by accident, but I have this high pitched whistling noise coming from the IAC.

I've searched and read up to discover the removal of the stock baffle and lid exposes this noise. The stock stuff was designed to cancel out this noise.

Short of reinstalling the stock stuff back on has anyone figured out how to get rid of this or do I have to just deal with it?

It's a NW 102 throttle on a FAST 102 intake. Stock throttle made the same noise. I had thought it was the alternator bearing for the longest time, until I left the IAC unplugged one day and the noise was horrible and plugging it in made it go back to normal

 

try taking something like sheet metal that you can bend and put under the lid on top of the air filter. you could even try cardboard just make sure it doesnt fray on the ends and dont forget about it.

other thing that comes to mind is using plastic epoxy to modify your new lid to mimic the oem one to reduce the noise

 

It's a volant CAI. There is no lid. I modded it so that it seals against the superhawk hood for a pseudo-pressure scoop. Anything I do would have to be to the IAC itself.