I have a 2006 GT. I just put an o/r h-pipe and mac axle back's on last week. Great sound idling and above 3500 rpm. When the car is under load and at low rpm's it sounds like there is an exhaust leak, but I have taken back to the exhaust shop and he says no leak.....his opinion is that its just the way it sounds and to put the stock h back on. It's an automatic and I have heard that auto's sound different than manuals. It really sounds horrible and I believe it sounds like an exhaust leak. This is my 5th GT and I have never had this problem before......anyone have this problem?? Any ideas?? THANKS!