Drive Train? - Not sure

There is sort of a clunking sound in rear of car as it shifts from first to second. Car is an automatic. Sort of a "thud" sound and******** when going forward. I need to have an idea what it could be so I am not taken to the cleaners on this. Some say drive train, others think gear box or transmission. I got this toyota because it is not noted for this kind of trouble. Bought it used 3 months ago and it did not do this. It was fine. Now I am terrified that it is something major. Thanks. Cherie