Expert help needed!

I have a clunk sound from the rear. It clunk when I drove at extremely bumpy round. It will only clunk once, not continuously .

I have shake aroun for loose parts or bushing. All seems tight.

I have no sound when I go in gear,floor the gas padel, stop what so ever.

Even on bump road it's fine, parking speed bump is fine, just on some crack bump on road.



What can it be

Thanks