That's moosing! If your car is still under waranty, have the dealer replace the IAC - which should fix the problem.
If your car isn't under waranty, you can do the TH fix, which took care of the moosing problem in my girlfriend's cougar.
Just a random thing I noticed - when I did the TB optimization, I noticed the butterfly has a very small hole in it. I bought a 3L TB (which I haven't installed yet), which I also did the TB optimaztion to... that little hole was plugged full of gunk. I wonder if that also contributes to the moosing noise.
Edit: I just noticed you have a 4 banger. I'm not sure if they suffer from moosing too.
Hope this helps...