Mazda BT-50: Automatic transmission problems

I just purchased a Mazda BT-50 3.2-litre diesel auto with less than 100,000 km and the auto gearbox is occasionally making a hard, loud clunk going from 1st to 2nd using sequential function and slips going through gears. Then today I started it and put it in reverse after a 1-minute warm-up and the gearbox made a loud grinding sound followed by a severe clunk when it finally went into reverse. I took it to the dealer and because the scan for error codes and software were ok they said just monitor it. Any ideas? I think damage has been done and it's not going to last?