the problem you have is when a mechanic places the car on a ramp, all the suspension components becomes unloaded. so to hear or see anything wrong would be amazing.
need to hear or look while the car is loaded, ie go to a garage with a pit.
no main dealer will have one anymore and unfortunatly they may say no fault found unless its cold.
try to explain the thud. in as much detail as possible and will try and help you out.
ie
is it on compression or as the shock is going back to its normal state.