I have aftermarket mufflers and I have this problem. I have checked and checked, but it seems like its just somthign that happens. Mine "pops" though because of the muflers I have. And Its exactly what you're talking about. Ive taken mine apart three times now and made sure it was on tight and it still does it so I wouldnt worry about it. BTW, how do you like the smell of your exhaust now. I got pulled over the day before yesterday cause the cop was, and i quote, "geddin my patroller stunked up".
I just said. "I used aftermarket racing fume injection", LOL gotta love that one huh?