Ford Territory 2013: Loud thumping noise

My husband has a 2013 Ford Territory diesel 6-speed auto. It had 73,000 km on the clock when he bought it around four months ago, and has now done 76,000 km. The Ford dealer has put in a new driveshaft, and new rear diff bushes. The problem is a loud thump under the drivers seat when slowing down over speed humps and when going around left-hand corners. Mechanics never hear it; they say we have to find it for them to fix it. We tow a caravan and worry this is going to let us down in the future. Any suggestions?