Is this a header leak or just the way they sound?

I installed the Mac LT's about a month ago. When I first installed them I know it had a leak. So I went and retightened everything. It sounded like a whistling sound when the engine running. I then went and installed the stage 8 locking bolts. The whistling sound went away but now I have a fluttering sound. I know that the bolts are all tight and that they are not backing out (since they lock) but I am still having the fluttering sound. I have uninstalled and reinstalled the headers 4 times along with the mid pipe . However, no matter what I do, I still get the fluter sound. Is this just the sound that the LT's make since they are flowing more air than stock. It does not make any unordinary sounds except when the engine is under load. Does anyone else have this problem? If so is it normal or what?