Hi .. You didnt say how many miles your car has covered.
.The problem might be the crown wheel in the diff ..When the bearings start to waer out the diff becomes loose and then you can here what sounds like a low rumble when you decelerate in gear.
.My has done 153,000 miles and been doing since it turned over 125,000. there is nothing to worry about as long as you keep a check on the oil in the gearbox.
.You could change it to a slightly thicker oil to make it less noticable or have the bearing changed which is not cheap when you add up the labour cost..
Neil...
February 2006