Loud banging in the exhaust while slowing down with the throttle shut is usually caused by an air leak where the exhaust pipe fits onto the cylinder head.

Weak mixture at small throttle openings can cause a less-loud banging or popping in the exhaust. The cause can be an air leak into the inlet manifold. Test by spraying WD40 or similar on the joints while the engine is idling. Other causes can be maladjusted idle mixture screw or a too-big throttle slide cutaway.

Amateur Loctite enthusiast.