Strange Noise In Exhaust At Higher Rpm's

Hey everybody I'm having a problem pinpointing a strange sound coming from my exhaust. I'm getting a metallic fluttering sound that starts when I hit about 4000 rpm. I've checked for leaks over and over and found nothing. I'm running pypes shorty headers, bassani catted x, and stingers. I just did cams but it was making the sound before hand. Now it's just more pronounced. I was also running muffler deletes before the stingers and i wasn't getting the sound. Seems like I can only hear it when I muffle the exhaust. Could it just be the raspiness from the bassani? Do some headers make this kind of sound? Could I have a warped header flange? Any suggestions would be greatly appreciated. Thanks

Hey man I appreciate the reply. After searching and searching for this mysterious sound, I finally found out what it was. The header flange was actually warped. Not much but just enough. Instead of just having the flange fixed I went and bought some replacements.I bought some BBK long tubes with their matching catted x pipe. Should have went with long tubes in the beginning. Oh well you live and you learn.