backfire sound

[scrappyk5]

when i shut my truck off , it kinda backfires through the exhaust. not really
big , kinda just a "poof" what is doing it ?

 

[southernspeed]

Some systems will do this no matter what. Exhaust scavenging in all to do with sound waves and if your system causes a bit of conflict when your on a closed throttle deceleration you can get that burbling/popping noise.
It is often also caused by an air leak in the exhaust or running a bit lean on the idle circuit.