Under deceleration, whether in gear or in neutral, the car has a fairly violent "thump" that occurs. You can feel the thump throughout the car easily, in the floorboard, etc. The thump slows as the car slows, so I'm thinking it's something that rotates. This thumping will stop if the suspension is extended and will thump if it is compressed. Both axles are good (new), engine mounts are new. Any ideas as to what's going on?

Car is a 2003 Mazda6 v6 mtx.