Almost everyone has a certain resonant "ringing" or chirping sound from the header...I really don't think it is a concern...the sound of the header/CAI/GReddy covers it all up practically....go ahead and get one.
Yes, I agree, the sound of the Greddy, especially, will cover up all other sounds in the vicinity. I figure all headers will have a resonant ringing at some point in the RPM range, though, so I'm not sure it is a problem. Or maybe I just want to believe that because my HotSHot Gen3 header has a slight hollow ring at a particular RPM, around 4500, I think. I can't remember exactly. Someone please correct me if I'm wrong.