Jerry,
Some times an exhaust leak might exagerate the rumble on deceloration. A common place to start checking is right at the head. The exhaust & intake nuts have a tendancy to loosen up on this motor with many people replacing intake/exhaust gaskets when they only needed periodic tightening. I've seen this often.
Then keep going down the line to the muffler & look for leaks or holes in the pipes & muffler.
Again, as stated by Vrod, some exhaust gargle is normal to the cars sound.