The Howl/Whistle is what I like to call it, I had a whole write up in my journal thread about it including links to videos with it... My conclusion is that the way the X pipe is designed it cant handle the extra flow that happens when installing HFC's or Test Pipes.... Just save yourself the trouble get rid of it and go with FI or HKS and combo up TP or HFC's or get the longtube headers and you wont be dissapointed
Here is link to my journal thread with write up.
*One_Quick_Z's Journal/Dyno/Videos/Pics*
DAN