FWIW somebody slotted their GT's airbox and (over on my how-to) said their car has a whistle now, that they like. I guess thats just the L36 war cry?
From your vid, I don't know that I'd enjoy that sound myself. My bud had a modded ECU and cone with box on his 03 GT and it made no such whistle. I wonder if you have an air leak in the new intake setup, whistling in a little unfiltered maybe?