I got a new Joe Z exhaust 3 weeks ago. It backfires (just a toot) when I get off of the accelerator, even at low rpms. But what is really bothering me is that it really stutters and pops and cracks shifting manually in Sport above 6000 rpm. It feels like I'm hitting the rev limiter (but I'm not). Is this normal or do I have a leak? I haven't been able to get it back on a lift to check it.