1998 Chevy Astro A "Backfire" type noise while at

Engine Performance problem
1998 Chevy Astro 6 cyl All Wheel Drive Automatic

when sitting at idle only ,my van making a popping or
backfire type noise,but not constantly. No loss of power. At the exhaust pipe you feel the exhaust
blow out but when it does this backfire, it feels like
it wants to suck it back in......HELP

Thanks, david