Heres what I'd do. Take a Bic lighter and run it around anywhere on the exhaust that there could possibly be a leak and go real, real slow. Check where the headers go into the motor, and check where your slip on exhaust goes on. Go real, real slow and if you see a draft moving the flame, well there's where you're getting air into the exhaust. I also believe these exhausts have a drain hole on each side to let out accumulated moisture. I'll go check in a second, but if those drain holes are rusted out to bigger than what they should be then you can assume your exhaust is sucking too much cold air and causing backfiring and popping and blubbing. That's what it sounded like to me, but of course I am NOOO expert! I only know what I know from the fellas here!