The amount of travel is really quite limited. At one point Morgan had a batch of very poor quality leaf springs but the current production should be OK. If, by chance, the leaf springs are too low there is the possibility that the top of the diff is hitting the tool tray, but that is very unlikely. In any case if the rear suspension is bottoming out you would really feel it.
If the sound is on compression, particularly in cornering, it could be, as others have said, the prop shaft touching the transmission tunnel. This would be a sort of grinding noise. If this is the case it may be that a panhard rod, a worthwhile investment anyway, would resolve the problem.
The other possibility is that the axle is hitting the plywood of the inner mudguard. These are sometimes cut with inadequate clearance and easilly fixed.

The other possibilty, if it is a sort of "clonk", is that as you come off the bump the bottom of the axle is hitting the rebound bump stop which is on top of the chassis under the axle. On my car I have reduced the height of this rebound stop so that it is at the minimum that ensures there is no metal to metal contact anywhere. A good set of adjustable damping rear shocks may resolve this problem as the rebound damping will help.

Peter

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