Better served in the Gen Tech section.
Are you sure it is not a belt "chirp"....you can remove the belt(s) to test this. Also while they are off check all of the pulleys.
When the ac is on, do you feel it or is it seen in the RPM's? Feel, could be mounts. RPM's could be a sign of either the IAC or TB needing a good cleaning.
Last, what is the exact code (P and four digits - P0420) you are getting?
66 Chevelle..78 F150..89 LXi 5 spd..97 SE Auto..01 Saab 9-3..07 Pilot EX-L
My child arrived just the other day, he came to the world in the usual way
Heaven is in the palm of my hand and it is waiting here for you, what I am I suppose to do with a childhood tragedy