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.
There is no tool tray fitted in my Plus 4 Sport.
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.
The sound is rather dull. Like something's going to hit wood. It is definitely not metallic and there is no grinding noise.
I've only heard the sound on rather straight stretches so far.
I currently have no way to lift the car. I'll try to get under the car as soon as possible.
As already mentioned the car is quite new. In any case I'll report this to the dealer at the first inspection at 1.500 km which will hopefully be achieved soon (depends on the weather).
