Having the same exact issue with my sr springs. Makes a popping noise only in reverse. I'm going to have my guy that installed them re-torque everything then go from there. I do also have the ford performance sway bars installed, maybe something needs to be greased?
Because around town you need torque and the only way the motor makes torque is when you are in boost lol and boost is bad for mpg. In the mustang im never over 20% throttle in town.