To me, it sounds like you are running a rich fuel mixture, and the extra fuel is collecting in either the Cat. Conv. or muffler. I would try to replace the O2 sensors. They tend to get lazy, and it is not a bad idea as preventative maintenance. It may save enough fuel to "pay for itself" even if it doesn't stop the back-firing. How is your gas mileage ?
2018 Equinox LT1 1.5 L 4 cyl, AWD, S/R Cajun Red Tintcoat/Black
2010 Equinox LTZ 2.4 L 4 cyl, FWD, S/R Cardinal Red/Black