Installed o/r h-pipe this past May and has flowmasters that were already on there. Anyways now whenever the car sits if I go to the gym or go get gas and turn the car off, when I start the car up the exhaust starts popping repeatedly out the left tail pipe (not as loud as a backfire but its noticeable), only on start up for a few seconds after you turn the key on. Any ideas? No exhaust leaks or anything either.