We had a clonking noise from our towbar (Mondeo, not a detachable towball, but fixed) and found the problem was just the tiniest amount of play in the bolts which secured the towbar fixings to the car itself. We investigated thoroughly, and somewhere on this forum are my responses to all the helpful suggestions which people put forward and the solution to the problem. Only after we towed another caravan with our towcar, and towed our caravan with another towcar, did we find the problem with the towbar fixing bolts and tightening them to the correct torque with a proper torque wrench resolved the problem.
All this was prior to our last holiday, and we had absolutely no problems afterwards, nor since.