[denisond3]

Im going to guess, since I dont know for sure....
At the forward end of the transaxle, where the shfter rod attaches, the transaxle is supported by a rubber 'mount' that bolts to the bellypan. If this tears from old age, it might let the enigine/transaxle move around a little bit on deceleration. While decelerating, the engine would be misfiring, as the mixture is pretty uneven when the throttle is shut. The misfiring is what can let the engine rock on its mounts, and that front mount is one that gets a lot of stress from acceleration, and might tear.
The other thing that you might be hearing is the muffled sound of the after-firing. (Backfiring is when the explosion comes up through the carburetor, If it goes out the tailpipe, thats 'afterfiring'.