I recently got a new custom exhaust. Was $150 for the new 2.25" pipe from first muffler was back. It works great and sounds good except when im downshifting and idleing down from speed. It makes a hollow fartty backfire noise. More like a popping noise. What would i have to do to make this not do this anymore. Im thinking im not getting enough back pressure... what can i do to fix this. I want the cheapest way of doing it as well... Other then that the exhaust is stock.